I'm trying to come up with reasons, at the margin, to bother reaching Plat and only come up with 2:

1. Higher priority for upgrades. Might matter for:
a. Routes that are super elite-heavy
b. People who buy tickets between the Plat and Gold upgrade windows

2. No change fees on award tickets (I think) and some minor things. Matters for people who need to make a lot of dummy or frequently-changing award bookings.

In all seriousness, I think there are a few people who would benefit from the above, but not that many.