It would totally depend on persons need and aspirations. Generally people would want to do an for following reasons
1) Field Change - suppose you are bored out of your field and want move in different roles like consultancy, finance etc
2) Earn More money post mba as you would end up working in higher positions
3) A person would want to develop network in the b-school
4) You want start up something of your own and you realize that you lack in some aspects of the business like you may be a very good techie but lack skill on how to setup marketing strategy etc.
While studying mba your would be studying a lot case studies from different segments of the industry which would help you later in life while decision making.