We have a new-ish development across the street from us that is mostly double townhomes. I think that’s what they would be called. Full-sized homes with attached garages. The garages are built between the 2 homes. They are owned, not rented and not cheap - high mid-range price. There are also a few single residence homes mixed in. I always wonder, what is the upside of buying a home where you are basically living next to someone like you would in an apartment? As long as you’re buying, why not buy your own house?
Because they are cheaper than buying a standalone house.
I suspect that this is the only real reason. Even if the townhomes in the OP are “high mid-range price,” I would suspect that the freestanding homes in that development are even more expensive.
Have you looked at the price difference between the detached and attached homes? I am guessing there is one. There might also be fewer HOA benefits with the detached. Every development is different.
I always find it curious on those HGTV shows, when they refer to them as “semidetached”.
Yes, I’m hardly an expert but my understanding is if they are truly considered townhouses (I refuse to surrender to the real estate industry’s complete replacement of the word “house” with the word “home”) then the HOA takes care of the entire exterior. You don’t have to do any mowing, pruning, trimming, or worry about the roof.
Duplexes
In addition to cheaper price, people may also be attracted to the new construction and the layout, both inside and out. Like, maybe the townhouses have postage stamp-sized yards that can be trimmed in five minutes with a weed wacker versus something that would take an hour to cut with a push mower.
Who pays for what can vary quite a bit, for both row homes and duplexes. In some cases, the HOA will regulate how you take care of the exterior, but you have to do the maintenance yourself (or hire someone to do it). It’s not part of the HOA fees. For very old row houses and duplexes, there might not even be an HOA. It’s fairly common on older duplexes to see different roofs, different siding, etc. on either side of the duplex.
That’s the usual term on this side of the water. One step up from living in a terraced house, and in their day a huge step up for the middle classes.
The difference from living in an apartment block is that you presumably get some outside space, and probably more than you would in a terrace…
What I’m not clear about the OP’s description is what’s between the houses on the side without the garage. Over here “townhouse” tends to mean an upmarket terrace - no gap between neighbours, possibly no or not much between the front door and the street, but high-spec interiors and something between a courtyard and a garden at the back. And presumably well enough built that there’s little or no noise transmission between houses.
I live in a townhouse and am responsible for all of those things myself.
I live in a townhouse, and I don’t have to take care of any of those things.
Oh, and in my complex townhouses like mine are now going for around $750K. You’d be lucky to find a house under $1.5 million in the same general neighborhood.
What’s a “high mid-price range” in your neck of the woods?
Yep, or the more sterile ‘multi family dwelling’. And a SUV is just a bloated station wagon. 
I can’t understand the appeal of the condo/townhouse experience, but that’s probably its own thread. But if I were forced into one, I guess sharing a garage/garage wall would be less obnoxious than sharing a living space wall.
I’ve lived (as a renter) in apartments, townhouses, and single-family homes (SFHs). Townhouses are more spacious and private than apartments (especially if you have an end unit, with only one shared wall), but cheaper and less maintenance than SFHs. As seen in this thread the level of townhouse maintenance will vary from “nothing” to “everything,” but generally there is less to mow, shovel, etc.
Aside from the shared wall(s) there are other downsides to townhouses – e.g., almost all of the ones around here are 3 levels, and therefore harder to consistently heat/cool than a 1- or 2-level SFH – but when the time comes to buy a place I could definitely see getting a townhouse (or a rowhouse, if I move to DC).
I like the idea of a no-maintenance condo, on a high floor with a nice balcony, but I’m way too paranoid about having noisy neighbors (or inadvertenly being one myself).
I don’t mind it. I grew up in a single family home, but haven’t been able to afford to live in one since. Other than not having a yard (that I wouldn’t want to have to maintain), I don’t really notice that much difference. Maybe we’re lucky we have quiet neighbors? Nice ones? But a little more space doesn’t guarantee a better experience IME.
Upon reflection I guess I would call 2 mirror-image homes, side by side, sharing 1 wall - duplexes/
I generally consider townhomes to be a row of 4 or more of them. My son and a niece live in that sort of thing. The benefit over a condo/apt is no upstairs/downstairs neighbor, and generally an adjacent garage/parking, and some outside patio/yard. I imagine most of those have SOME sort of HOA for maintenance of common areas - mowing grass, shoveling/plowing walks/parking. Imagine such things as roof and gutters might be common as well.
Is that what would be called a “terraced house” in London? In movies such as Billy Elliott, you see rows and rows of apparently connected homes, with no space between them, and apparently walled back yards.
In some very up class areas of Chicago - such as the gold coast, very pricey homes are right next to each other, but they differ in style/construction/etc…
My parent’s bought one unit in a duplex after they retired. Well, I would call it a duplex, but their real estate agent insisted that it’s not a duplex, it’s a “twin home”.
Just like they’re no longer called trailer houses, but manufactured homes. 
I grew up in a neighborhood with pairs of houses (no garages) and we called them semi-attached. We shared a wall with our neighbor and there were driveways on either side. The houses were built in 1951, and were common in Queens. Plenty of yard, and it was not called a townhouse. The next block over there were rowhouses, with garages in the back on an alley and little land. They were smaller and I assume cheaper.
Nothing new about these. My wife grew up in something similar, and that house was built in the 1880s. It still had a shed which used to be a stable in the back.