There are 3 types of garrison buildings, trusted I can count.
The first is the Bunker. Its small, and not too expensive for when you unlock it and that mission that requires it. Still, there is not much use for it, and since it takes up population, i sold it after the mission.
Then you have the guardhouse, which is moderately expensive, also looks cool, yet is still useless.
Then comes the later missions where you have to build a Garrison, which is very expensive and costs a ton of materials to build.
It took me forever to finally get the materials to build it, and once it was built, I had no use for it other than completing the mission. Still, i dont want to sell it since i spent so much on it.
As it stands, Garrison type buildings are the most useless in the entire game. They serve as overpriced decorations that eat up your manpower and give you nothing in return.
Sure, one could argue that when PvP and raiding comes around they might be useful, but who actually cares about getting raided? And since PvP is not in the game yet, these buildings have zero value outside looking cool. If they didnt take out manpower, id keep them around.
However, I thought of a solution to making them at least somewhat useful: when NPCs (Raiders, Rebels, Mammoths, etc) attack a square witin the defensive area of the Garrisons radius, you should be able to deploy a troop from the Garrison into the battle, but it doesnt count as 1/7 Units and instead counts as 1/2 defenses.
This would give the player a huge advantage for when a defense type building is not effective against the present enemy formaiton. It would also give players a reason to build and keep garrisons around, as well as spend more time training troops and vehicles they would want to use in these battles.
Right now, garrison type buildings serve zero purpose outside looking cool, so if this seems too OP, id at least suggest removing the population usage on them so we can use them as decorations without shame.