The reason why people would want to set up one computer to store files for another computer are varied. For example I backed up my Linux box (PC) to a Mac before the power supply died. In my case the choice of a Mac comes down to two reasons. Ease of set up and stability.
A common access method is to use ftp (file transfer protocol – note the lowercase letters). In OS X go to the System Preferences and look for Sharing. Depending on which version of OS X is running the selection of the next category may be different. In Snow Leopard (10.6) the selection is under File Sharing. Individuals may want to tweak the settings here depending on the version of OS X. The Mac can be accessed by computer name or IP address (found in the Network preference).
There are other methods than the use of ftp. I have found for cross-platform use ftp is best. Settings for SMB relate mainly to Windows style sharing. AFP seems to work only between Macs (from the Finder –> Go –> Network or Computer).
Two items of note… First, depending on the version of OS X, the size of the hard drive may be limited (thereby limiting the potential storage area). Second, in Tiger (10.4) only the main hard drive was able to be accessed for storage.