Mac OS has a built in tool called XProtect, that is updated by Apple. This blocks or removes some prevalent Malware/Adware. It is most similar to Microsoft's Malware Removal Tool they publish and run with each month's windows update.
XProtect and MRT neither have a real-time scanner, which is where Avast comes in.
While true that there are far less malware threats on the Mac than on the PC, it is far from invulnerable and new threats are seen all the time for Mac.
