Is it to simply have the experience of being on the other side of the relationship?
I think this is a hugely important reason why therapy for students in clinical programs should be strongly recommended, if not required. I can't help but think it makes it far easier to empathize with your patients as to what it's like to be therapy patient if you've been one yourself.
I also don't want to dispense with the idea that there are unique opportunities for role modeling available in the therapeutic relationship. I don't mean patients picking up therapy techniques from their therapists, but just learning how a professional acts with a patient.
In both cases these are forseeable benefits of students getting therapy as part of their training that are probably hard to quantify in terms of objectively verifiable outcomes. And while I agree that having training of psychologists be data-driven and empirically-validated at all levels is a great ideal, I think it's also an ultimately unrealizeable one. If we applied that standard to everything that goes on in training programs, there would be quite a lot of training activities and requirements that people consensually agree are quite helpful that would get dispensed with. Maybe that's what we want?
Students who go to therapists as part of their required (or suggested) personal therapy and then use the time to discuss cases and countertransference have some serious issues with misunderstanding their respective roles.(therapists and students both).
Finally, to end on a provocative note, I do wonder a little bit about some psychologists / therapists-in-training who are particularly, personally, viscerally opposed to the idea of receiving therapy as part of their training (and no, I'm not speaking of any posters in particular, so calm down). You have no issues, literally,
no issues that might be useful to talk to with a therapist? Really? "The lady doth protest to much, methinks"? Therapists work with resistant patients too - and it's often quite productive to do so. Maybe here as well.