What is the point of capturing outposts?

Capturing a base for the first time unlocks a word you can use as part of your emblem design. It also grants some heroism. Subsequent captures only grant heroism. Beyond that, it's simply a means of telling you there are no more hostile targets in the area, which can be nice to know if you'd like to destroy or extract equipment with impunity.

Note that is applies to all enemy installations of any complexity, not just the small guard post stations. You can capture villages and forts as well.

The bases will return to fully-staffed status once you leave the map. They can also become staffed again if you only captured the base through non-lethal means and the guards wake up. They make also repopulate if you leave for a long time and return (without leaving the overall larger map; I've not tried this).

I don't know if you can "capture" a base for heroism bonus more than once on a given visit to the map; if you can, it's certainly on some kind of timer because you can't farm heroism by constantly waking up and knocking out the last guy standing.