The software runs on lowest priority. So when people are using the computer the software just sits there, not doing much. After a few minutes of inactivity the seti software (which sometimes was a screensaver) would kick in. Rather than sleeping or idling the computer would be doing computationally intensive work. Some distributed stuff (especially Prime95) is used specifically because it makes the computer work hard - it's a reliable stress test. This causes extra power to be used for the processor and the fans.

http://www.mersenne.org/download/