Diagnostics are dumb in general, in my opinion -- it's hard to get a sense of where you stand until you've spent some time understanding the exam in general, and your initial score can be very unrepresentative of your final score. I therefore find taking a timed diagnostic unnecessarily limiting and discouraging from a pyschological perspective.
You do it so you can brag about how much your score went up.
Honestly, I think test prep courses push these so THEY can brag about how much your score went up.
I never have students do this if I can help it.