Ofc you don't but you have to use a parser to know: Why did my dps increase in this fight? If not, why did I do less? Was it less critt? bad rotation? Bad hold cds etc? Good popping cds etc? If you do a fight and lets say you don't run a parser, someone ask if you did good dps and you are like hey, I did more crit so I did more, but in reality you might done less because of error of rotations.
And I will agree with some people here, a parser being official on this game, won't make players better, but at least it will give them awarness that they aren't as good as they thought they did. Some people will react posetive to it, by seing those ''numbers'' and data and see what they are doing wrong or better than others. I can honestly not count how many times I've went on FFlogs to check other BLM's rotations to help my friends rotation. There are times I've done 9 min v1s fights with pugs and only seen NIN do 3 trick attack instead of 8 -9. The parser gives you info on that too and many ignore that because they all think we say ''blabla numbers'' It's not the case really, parser is much more than just numbers.
Keep in mind, those people who say parser will be a huge toxic fest if it ever happens, well ever looked how currently players are? There are far more toxic players without one. In fact there are even people who calls people bad yet they are the worse ones.