Hi,
Imangine this is the scenario
r1----bgp----r2-----bgp----CE(r3)-------PE1---------------------PE2------------CE(r4)----------r5-----------r6
Now r3 who is the cutomer edge router has already his internal network setup , how will PE1 vrf know about his internal network routes without running dynamic protocol, sure you can write static routes but that isnt managable, hence you need to have a dynamic routing protocol which gets all the customer routes from CE and then redistributes them to PE bgp process.