For the reasons I said in the same post? It's helpful. I can know that I'm doing "enough damage to clear the encounter by some margin or other" or I can know exactly how much dps I'm doing. One is enough information to get by, one is even more information. It saves a lot of time and effort and helps quite a bit to optimize.
So in short, yeah... just the exact reasons I already posted?
Edit: unless you're asking why I specifically want an in-game parser rather than 3rd party? In which case I don't really care tbh. The third party ones work fine. Whether SE adds their own, or just updates their ToS regarding add-ons, I'm happy. But it's really awkward to be breaking the ToS just to know how much dps I do as a dps class.