Do you believe running your computer for 24/7 running Boinc, examples, seti@home, rosetta@home... is worth the extra power bill? Sorry if this is in the wrong section!
The idea is not to run your computer solely for all these, but to let it run in the background for those times when you are just browsing the web, checking your emails, and also those lunch/dinner/toilet break when you leave your computer on.