Why do computer systems need a main memory when programs and data can be

Computer systems need main memory because it is faster, dynamic, and closer to the processor. There is actually a part of your hard disk that is sectioned off as virtual memory just in case you run out of actual memory.

Exactly. The hard disk, by technology, is still the fastest storage medium around - as compared to diskettes, cd-roms, thumbdrives and what have you. Let's think it from another perspective. Would you want to open your drawer everyday to load each program you need to run? Further, some programs are so big that it is just not possible to store on disks - and you might not have access to a larger capacity cd-rom or thumbdrive.