I can not for the life of me figure out what the purpose of the little pocket sewn into the right-hand pockets of my khakis. All it does is confuse my fingers when I’m fishing for around for change at the checkout.
You put your change in it. Keeps it together.
In days gone by, this was for a pocket watch, and the tradition has continued.
I have a ponytail, and it’s ideal for holding ponytail rubber bands. I get really annoyed now when my pants don’t have one.
I put my vitamins in the little pocket.
I thas whatever purpose you decide it to have. It’s origins might have been to hold a watch, but this hardly means it has no purpose other than a watch.
Imagine these two scenarios:
Hate mail received by mfgr who continues including the little pocket = near zero
Hate mail received by mfgr removing the little pocket = lots
**
Sometimes **marketing is brainless.
Condom pocket, Duh! 
Or, if you’re (literally) penniless, your cell phone. My cell’s a perfect fit in every pair of pocketed-pocket pants I own; the pocket keeps the phone stable and vertically oriented, which feels, looks, and works better.
I can barely leave my house without my good ole Swiss Army Knife. That’s where it goes.
And - the makers of the new Dockers trou eliminated them! (Fortunately, they made up for it with me by making pants with a slightly expandable waist band, which, ahem, comes in fairly handy in my line of work, and with my own expansion).
I think we may be getting confused about two types of pockets. I believe a fob is generally a pocket that’s accessable from outside the clothing, a bit like that fifth pocket that are on Levi’s 501s, and is pretty much a holdover from the pocketwatch days.
But then there is inside, and at the bottom, of the right pocket of most dress slacks. That is there so that your change does not clink and slosh around as you walk. I do not know if there is a special name for the pocket, but I do not believe that it is intended for a pocketwatch. This, I believe, is what the OP is describing.
Am I the only smoker in this thread? That pocket is perfect for holding a lighter.
It’s also perfect for searching through when you’ve run out of cigarettes and you’re hoping you accidentally stuffed a ten in there somewhere along the way.
That inside bottom pocket is where I put my keys if I don’t have a coat. Keeps 'em from jangling around when I walk.