Better software compatibility with the 020 chip. Dose not have a built-in MMU, like the 030 chip. The 020 chip uses an external MMU chip that can be turned off for better software compatibility. 68020 boards are usually made better then 68030 boards.
The MMU in an 030 is generally not used and is off by default. It doesn't affect compatibility in any way. The only things that might affect compatibility are the data cache, which can be disabled, and the missing CALLM and RTM instructions, which were very rarely, if ever used.
