Paint jobs change the colouring of a robot. Currently only Schutze does not have any paintjobs on the Live server. Even though at times they may act as camouflage, this is in fact useless as people can plainly see your name tag and lock on to you. Paint jobs have no purpose beyond cosmetic purposes and flaunting wealth.
Some of the skins cost as much as 2500 gold, some as little as 50. It is unknown why some bots have 2-3-4-5 skins, and some have only default. Generally, it seems that most popular and\or gold priced bot receive skin, which seems logical - if player bought gold bot - he possibly can afford skin. Or, if everybody is running same bot model - you might want to differentiate yourself.
Most popular opinion, though is that the main appeal of paint jobs for people is customisation beyond the weapons and onto the robot body itself. There is no real strategic point otherwise to get people to focus on you more, due to them viewing you as a robot that stands out in certain maps.
Alternative opinion is that skins can help to see which teammate has all of his weapon upgraded (judging logically - you must have a LOT of money to purchase skin, so you already spent gold on everything you could - like buying weapon and speeding up upgrades).
Skins that are NOT available for purchase
There are some very high-quality fan-made paint jobs on Pixonic's page; however they are not in the game for some reason. Some of the examples of such paintworks are at the bottom of this page.