Rotoscope artists look at film frame by frame, 24 frames in each second of film, carefully tracing around individual hairs or hoodie cords so that someone else can have the satisfaction of putting the actor or object behind or in front of explosions or dinosaurs or Jar Jar Binks. For the rest of their lives, roto artists are thus instinctively drawn to people who wear tight clothing and have short slicked-back hair, so there's a pro tip if you're looking to date one.
Why would someone want to trace Brad Pitt's head for ten hours a day? Well, it's a stepping stone to the cooler jobs (like Compositor, the guy who gets the satisfaction of actually pasting different elements together into a frame that actually looks like something). Roto is an initiation stage, like a fraternity pledge or a knight's squire, only if most pledges and squires never got promoted.
The second reason you'd want to do roto is if you live in India. A lot of this grunt work gets outsourced these days, because it looks pretty good if your only alternative is to have your eyes put out by a red hot poker and sing for your money, which is what I gathered about Indian career options from Slumdog Millionaire.