Do I need this subscription?

First of all, the $0.99 per month gets you 50GB of storage space, which is 1,000 times the storage space you indicated.

Secondly, you have rented iCloud storage. If you give that up, you will lose whatever is there that exceeds the free 5GB Apple provides you. Certainly iCloud storage space is not interchangeable with AT&T storage, but if you carefully and judiciously manage your storage, you may be able to make it work.

Manage your iCloud storage - Apple Support

When you cancel the 50GB storage, you will lose any data over the 5GB free level. Since you have storage with ATT, store whatever you need there first, then cancel your iCloud storage plan and go back to the free level.