Three reasons:

The SE replacement shots are from a release print which has grain. To make these blend better with the disney main source you add grain to the whole film.
The movie was shot on film. Disney went crazy with the DNR to the detriment of the movie. Add some grain back and it looks better and more authentic - this is a theatrical reconstruction after all.
Grain is good generally (I’m talking negative grain not print grain - that is totally subjective).

If you don’t like grain then stick to watching the prequels. Or as I said earlier, make your own version with no grain.