If you're hitting a spending threshold when making a purchase, you're effectively getting more ppd (point per dollar) of spending.
So, for example, a metro card purchase of $100.
With the Sapphire Preferred you get 2 ppd on travel (so with the with 7% dividend): 3.75%
-Sapphire Preferred without 7% dividend: 3.5%
If URs are worth 1.7 cpp (.017 cents per point) you're earning 6.375 or 5.95 cents on the purchase.
If you're earning a bonus on another card though, that purchase is in effect earning more than 1 ppd.
For example the Delta card. 50,000 points for spending (IDK) $5000 (?). Then
50,000 points = ??
$5000 $ 1
or 10 points earned for each dollar of spending.
Even if each point is worth only .01 (1 cent) it's still more than what is earned with the SP.