How do you use Reinforcements?

I researched the Reinforcements ability but I can't figure out how to use it.

The in-game description states the following:

• Respawning Marines are dropped on the front
• Respawn speed is 30% faster
• New Marines are inspired to fire 300% faster
• Each heli-drop costs 1.5 ammo

Once I build the structure though I just... sort of end up with a building I can't figure out how to interact with, as seen below. How do I actually call reinforcements? I have no idea how this ability is supposed to work, does it increase my marine count over time somehow?

All of the benefits of the Reinforcements building are passive: you can't activate any of them via commands. Going through those benefits in order:

1. Normally, when your marines die, they respawn from a base after a while. (I think it's the base closest to where they died.) You can see their health counting upward in the HUD at the top of the screen - when it reaches full health, the marine respawns. Now, when your marines respawn, they'll spawn roughly where they died (usually where there's still fighting going on!). They don't need to hike across the map to get back into the action.

2. Respawn is 30% faster. So when your marines die, they'll both be back sooner and arrive where you need them. In total, they get back into the action much faster.

3. Newly-respawned marines fire faster for a little while, making it much less likely that they'll die again. It's a lot like having an Officer around when you destroy a hive: more bullets in the air means fewer aliens menacing you. So when your newly-spawned marines arrive at the place where they died, they'll reinforce what's left of your troops with some rather nice firepower.

4. Each time you get reinforcements dropped off by heli, it costs 1.5 ammo. I haven't used the Reinforcements building terribly much (I pick up Grenades first, then some weapons and Engineering; by the time I get around to building Reinforcements, I'm already winning), so I'm not sure what happens if you don't have the ammo – I would imagine that you get the marine back, but they spawn at the base instead.

So: Reinforcements is about turning marginal losing battles into winning battles, or allowing you to continue to march forward as you win costly battles. As marines die, they come back fairly soon, and they come back with heavy firepower. It's better to never have anyone die, but that's... not always possible. Reinforcements won't help with badly lost battles, though – if your squad is almost-all-dead by the time your reinforcements arrive, the new marines will be just as dead soon enough.