In contrast to the other replies you've gotten, I think my habit of highlighting is more of a nervous tic than a conscious decision to help me read. The habit is certainly related to my tendency to play computer games that require a lot of clicking (League of Legends/Starcraft), but if memory serves correctly I've been doing it since before I started playing any games. Also, I'm the kind of person that will sit at my desk with a pen clicking and un-clicking it endlessly before I remember that other people are annoyed by that kind of behavior.