They are used in a lot of the deer feeders that hunters use to attract deer to their stands. All the hunting stores carry them.
Yes. A cell’s chemistry determines its voltage. Any carbon-zinc dry cell or alkaline will have a nominal voltage of 1.5 volts. The larger the cell, the longer it can deliver that 1.5 volts.
Wait, wait, wait. Isn’t deer feeding by hunters illegal just about everywhere?
Baiting deer on private property is legal in Texas. Cite. I don’t hunt here, but from what I gather it’s very common to pay private landowners to hunt on their land, and that the landowners use the feeders to ensure a supply of healthy deer and keep the income stream flowin’. They sell the feeders, along with huge sacks of deer corn and game cameras, at places like Cabela’s and Gander Mountain, and even Academy Sports and Wal-Mart.
If you open up a 9-volt battery you’ll see that it’s made of 6 smaller batteries wired together – sized A I think.
The cells in a 9V alkaline battery are six LR61 cells wired in series (1.5V x 6) which are pretty close, but not exactly the same size as AAAA cells.
Backing away to the meta-issue …
As e_c_g had it in post #4, they started at A and went upwards as things got bigger. Then, surprise!, there was a need to go smaller than the original origin value “A”. Batteries are far from the only size nomenclature where this has happened.
I wonder how much this is an artifact of 1800s / early 1900s thinking. In those days, each new tech advance was bigger, more powerful, etc. It’d have been easy then to have the mindset that the future will necessarily only be bigger yet.
Something fundamental changed in the 1950s with the advent of the transistor and the jet engine. Suddenly we could increase density and put more “power” (in whatever unit of measure) into a *smaller *package.
It seems obvious to us now that miniaturization and improving efficiency are the way of the future; cramming more oomph into ever less area. And meanwhile, as the tech matures, also making ever bigger things composed of ever larger numbers of more efficient and powerful but smaller components.
With all that in mind, have we seen more sizing nomenclatures defined where the expected direction of growth is down rather than up? Or more that are inherently double-ended and can grow in either direction without kludges like AAAA batteries or 00000 gauge wire?
Or have we simply walked away from these arbitrary index-number-as-name approaches and gone to simply labeling stuff by its key operating parameter: e.g. 10 amp wire or 1000AH batteries or …
9V batteries vary. Some are cylindrical cells side-by-side. Others have stacked rectangular cells. Cylindrical LR61 types are probably the most common today. Stacked rectangular cells were more common in older carbon-zinc batteries.
I found some pictures of the different types on this wikipedia page (about halfway down the page on the right):
Depending on your state, you may be able to feed up until a certain time before the season opens. Our you may be able to feed during the season, but only so close to your hunting area. And some of my neighbors feed just because they like to see deer.
and 0 for round. 
in Michigan you can do it, but it might be coming to an end as an effort to contain chronic wasting disease CWD.