Transmitter

This building is required if you wish to join or create an alliance. It also has an option, if you are in an alliance, to request reinforcements. This posts a clickable message in your alliance chat area.

Upgrading allows you to receive more units from allies as reinforcements, and to send larger (i.e. they take more space) units to allies when they request reinforcements. Reinforcements can be used in any attack instead of some or all of your own units. As they are 'used up' by this whether you win or lose the engagement they are excellent for use as suicidal distractions or as bait to clear out Detonator Droids or Phantoms.

Changing Region
Your Transmitter is also used to Change Region, which effectively moves your entire base from one Martian zone to another. Some zones are PvP regions, like the one you start the game in, where you can score points on the PvP ladder by destroying 50% or more of your rivals' bases. Other zones have quarry areas which, if you can conquer and hold them, produce Water and Iron in much the same way as the Harvester buildings. The 'Collect All' button collects the resources for you.