Z T codes for standard Transactions

Hello,

I have seen in many projects that there are z t codes created for standard T codes. For example for creating sales order , VA01 is used but in some projects ZVA01 is used.

if authorization control is the reason then that can be controlled through Authorisation profiles by using standard T code VA01 only.

why the need for ZVA01 or any Z t code representing a standard transaction.

regards

sachin