In every home connected to the internet there is at least one computer.
In every computer there is at least one CPU.
Everyone of those CPUs are cycling between 300 MHz and 3 GHz.
If just a few of those cycles are not put to use it seems like an awful waste of cycles.
SETI@home and Astropulse are funded by grants from the National Science Foundation, NASA, and donations from SETI@home volunteers. AstroPulse is funded in part by the NSF through grant AST-0307956.