For example. I bought some plastic skinks, cut their crests off and painted them earthy green as opposed to aquatic blue like me regular skinks. I use them as chameleon skinks because they are cheaper and I don't like the GW ones.
I also converted a PP swamp troll into a slann, because it looks like a giant angry frog. It fits on a 50mm base and looks like what its supposed to be.
So basically, if you convert something, you use it exactly the same as you would what it supposed to be. It just looks different, that's all.