Basically it would turn the current selection (of the same units) as a buildable selection. So if I want to build an energy node, I'd just select a nearest node, press that hotkey and plop them down, instead of going to the menu on the top left. It may not sound much, but plopping nodes and cannons is a standard routine on most of CW3 maps, so cutting amount of clicks will reduce the amount of busywork substantially.
I like it...
This would also allow for constructing a formation of relays and panes for use on flat surfaces to quickly construct it. As Virgil said: I like it.
I would love a duplicate hotkey. I hope this gets into the final game.