The R2 units repair any ship/vehicle you are in when you are near them... there is the usual tell-tale chirrup from the unit when it is working.
Now for the abuse.
Having problems with AT-ATs and other vehicles?
Not a problem! Just find yourself a lovely little R2 unit and slap your vehicle in front of it.... this works peachy on a couple of maps at least! Then those pesky vehicles can pound away and your vehicle will hardly ever blow up!
It also helps if you are a pilot (so you regen the ship too), and if you can get a 2nd in the ship (hammer that f2 key), and he is a pilot even better!!
On Theed: plains & um the snow one with at-ats that isnt hoth (I forget the map name), you can use this to great affect and bag yourself well over 100 kills per game.