I don't think anyone's come out and said that a parser is necessary. Plenty of people have said that it's a useful tool, though. People that use parsers are generally the type of people that like having more information about things. Our character stats grow and change, and it's much easier to make adjustments to your rotation (and know if those adjustments are beneficial) if you can parse the damage you're doing. It can help you with equipment as well, as far as choosing which secondary stats are most beneficial. Do you think the optimal rotation was just suddenly a thing that everyone knew how to do? People most likely sat there for hours with parsers going over different rotations and seeing which was the best for them.