Regarding Message Type in ALE

hi guys,

i was involved in Master data distribution using Message Types MATMAS, DEBMAS and CREMAS but never created a customer specific Message Type that is why, i just want to know in what cases do we have to create our own message type's using we31, we30 and we81 and we82.

can any one out here help me out by sharing giving some real time scenario's in what cases do we exactly have to create our own message type.

thanks

Pavan