Why do we need different PGW for different service?

Question

However all PGW doest same job like forwarding IP packets,IP allocation,TEID maintenance then why do we need separate PGW?

Answer 1

Each PGW provides specific type of service for many thousands of users. If an operator starts giving all services through same PGW, network congestion and load would increase which may result degradation in network performance and users experience may go for a toss. For better service management, operator would prefer to provide each service through separate PGW.
PGW also support many interfaces for mobility and charging functionalities. This is also a reason to keep separate PGW for each different service.