The only way to build combat-unit factories (Bot, Vehicle and Aircraft plants, Shipyards)
Besides building, it can repair, assist (speed up the construction) factories and directly build mobile construction-assistance & scout units (Bow FARC and Kern Muskrat)
Outposts start at Tier 1 and can be morphed up to Tier 5, when they're able to build the cutting edge Gantries, massive structures capable of building Behemoths and other super combat units
Outposts spend energy for merely existing, so unless you're floating (wasting) energy it's usually not a great idea to "spam" (build many) of them to assist (speed up) construction. Initially FARCs are best suited for that role, at least until they start clumping and disturbing unit production due to their short synthgun range.
Besides building, Outposts can repair, assist factories and directly build builder units and mobile construction-assistance & scout units (Bow FARC and Kern Muskrat, generally called "FARCs")