FAFSA for US PhD (hard sciences — physics) students?
First off, while many PhD students in many degree programs in the U.S. receive TA and RA positions, these are hardly universal. Many students, especially those who are foreign-born or in the humanities, are responsible for paying their own way.

You need to complete the FAFSA if you want to be eligible for student loans, which could help bridge the cost of your living expenses if your TA/RA stipend isn't large enough. For some universities, it can also unlock the door for scholarships or other funding that may cover things your TA/RA does not. It's not uncommon for TAs/RAs to still have to pay several hundred or even thousand dollars per semester in fees.

If you're a first-year PhD student, I'd suggest completing the FAFSA and see if it yields anything beneficial. After the first year, you can decide if you want to keep filing or skip it.