Is the memory scan nessecary? What exactly does it do? An explenation would be appreciated.
There are many viruses that load their code into memory so when avast scans memory if it finds a virus it will notify you and suggest you run a boot-time scan to try and deal with it before windows starts and the main virus loads it into memory.
Memory scans can detect ‘active’ infection. Active virus are, generally, more difficult to remove than the non-active ones.