We have 3 computers in our home and leave them on all the time. The primary reason is because we are partnered with United Devices which uses our computers idle processing time to help create Grids in finding cures for various diseases. Here is a blurb from their site:
Grid MP consists of a server and agents distributed to designated machines (servers, clusters, or desktops) that can be in the same room or on multiple continents. The software schedules processing jobs to machines that "advertise" their availability (meaning they have idle processor time and are equipped to perform the necessary computations) – ensuring work is scheduled to the best-suited resource. The machines return results to the server, which recompiles the data and delivers it to the user in an easily searchable database format.
We have been making our computers available for over 3 years now, but the total time registered on our account in terms of years is over 14 years. For example, mine is working on a cancer research project right now. You can read about it HERE. Just something to think about if you don't already know about it.