I agree – if applying for funding or an EHCP you will need the letter on file as evidence. They can also be very helpful and may give extra information. For access arrangements, having diagnosis evidence removes the need for much of the testing process, so also very useful; we had to make one pupil sit a whole set of tests because her mum couldn’t find the diagnosis letter.
Obviously, for the pupil’s learning, it should make no odds as you support need, not diagnosis, but for anything official it does help.