I have googled, I have searched here, I have posted elsewhere, I have combed the strategy guide. I can't find even a partial list. I can occasionally find mention of a couple of them, but other than that they seem to be ignored. Once you get a companion or two they become less of an issue, but the completionist in me wants to find more of them and even with companions I still find them useful.

I have found 4 so far.

Goodsprings - On the porch of the General Store
Primm - Inside the Mojave Express building (go figure), also the Nash's residence (same building).
Novac - Inside the motel lobby (convenient for safe house access)
Freeside - On the street corner outside the Old Mormon Fort

Has anyone found any more? I've been keeping an eye out, but I haven't noticed any others. I've been to the strip, but was looking at other things and haven't extensively searched the various casinos.

There is only one more, in the Las Vegas Boulevard Station.

what are they for?

You can use them to mail yourself stuff. Once you have discovered at least two of them you can open them up and choose to deliver stuff to another location that you have discovered. They are handy for offloading junk you want to keep but don't want to sort out right now. They are superseded by companions carrying capacity mostly, but not entirely.

Personally I use them when both me and my companions get full-ish and I don't want to interrupt my quest flow to go back to the motel room to sort and store my junk.