Mine is just a particular case, but as a mathematician I was attracted to computer science (as a legitimate field with important real-world effects) thanks to a talk that RMS gave in my university. First I liked his personality and sincerity in the way he explained the printer driver story; then I listened and read everything else he said (especially "the right to read") and it ressonated with my thoughts very powerfully. I guees the same thing happens to many people.