DD refused to apply for DSA before she started uni (on the assumption that by getting a place, she would miraculously become well
), but she had to provide medical evidence to the uni when she firmed her place, which triggered priority booking for halls, setting up of exam allowances (rest breaks, opportunity to sit exams in the resit period but as a first sitting), automatic extensions for assignments.
The DSA assessment was great - all about "what stuff can we offer you to make your study easier?", and equipment she can keep afterwards. Not sure if the software needs to be relicenced each year, or if that's for always too.
In DDs case, the application processed was smoothed by already getting DLA, which made the medical evidence needed less important (fortunately, as she hadn't seen her consultant for nearly two years at that point).
Her friend, who applied in advance of starting, had all her equipment and training provided at home in the summer term before she started.