One guy I worked with would do it to protect his bailiwick. He was intensely territorial, and knew that writing code so ugly that nobody wanted to even look at it, much less take the time to understand it, was an effective way to make sure that nobody else on the team would ever touch anything he wrote.
Sounds like "mortgage code"... code that is intentionally so complex that noone in the company other than yourself can maintain it, hence you end up with a job for life that pays your mortgage.
Ha. Unfortunately for him, it became clear that most of his code was all sound and fury, calculating nothing. Well, that and he took forever to fix bugs because he couldn't really understand it either.
He was soon out on the street, and the rest of the team's policy of just rewriting bits instead of debugging them (it never took very long, and frequently resulted in 1/10 as many lines of code doing twice as much) had soon swept away most of his footprints.
