Its easy to build a Linux based PVR (like Tivo) which can record and play TV programs. This is useful if you don’t want to or can’t be there to watch your favourite program, so you can program your PVR to do the work of recording which can be watched later.
MythTV is a popular open source solution with with you can use a PC as a PVR.
Clever Tricks with MythTV is a nice article which explains how to build a PVR quickly.