To use the value in your P2P app you have to convert it into a card. This process is cumbersome and time consuming.
Carrying and using cards has inherent risk today. Cards are lost or stolen and a persons identity can be compromized.
Being able to use your favorite P2P app to purchase goods at any store is far more convenient the converting cash value to a card.
You control the authorization of the transaction. The network does not need or want your personal information. Just your approval to perform the transaction.