Its mainly used to get an enemy off a station dock point, or gate jump in point. This is best accomplished by a big ship going fast, I have seen Machariels (fast faction BS) bump guys like 30km off a gate.
The generic way of doing it is with MWD and Approach. Sometime you need to bump a guy before he gets back to a gate, so you have to pick a line that would intercept him. Sometimes you have to get into position so the angle you run into him at will correctly get him off the gate, where you can web and own him.
"I have more qualifications than Jesus and earn more than this whole board put together. My ego is huge and my modesty non-existant." -Ironwood
Its one of the few pvp tactics you can train all day long in empire without getting in too much trouble
As long as you don't pick on a freighter
QTF, you can be successfully petitioned for harassment for doing this to a freighter.
bumping itself, it's about mass, it's used to get you off warp/dock/remote repair range and of course to desync your supercapitals. Though CCP pretends the latter never happens.