Am I missing something here. And if I am not why would anyone charge anything to a Delta Credit Card?

Delta Gold - 2x miles on Delta Purchases
Delta Platinum - 3x miles on Delta Purchases
Delta Reserve - 3x miles on Delta Purchases

Amex Personal Platinum - 5x Points on flights booked directly with airlines.
Amex Personal Platinum points can be transferred to Delta at 1:1 ratio.

So a $1000 ticket purchased on Delta Platinum or Reserve nets you 3000 Delta Miles. The same ticket purchased directly from Delta on the Amex Platinum gets you 5,000 Amex points which can be turned into 5000 Delta miles, or used for other purposes.

This takes nothing into account that the Amex points can also be transferred to many other partners as well as used as cash equivalent via their travel portal hotels, flights, and rental cars.

I'll keep the Delta reserve card for the annual first class companion ticket but as far as actually using the card why would anyone do that when you can earn more points/miles, and more flexible points, by using the Amex Platinum card? Yes Delts reserve is a tie breaker on the upgrade list but now that is below million miler status so it is less valuable.