The DEV server does not contain much data and the Unit Testing is normally done there.
There can be several changes done to the DEV system that are not imported to PRD (E.G. Local Objects or transports not yet moved). The QAS system is an exact copy of PRD with less data. Quality server is the box where integration testing happens. Business and super users usually do testing here, before it’s moved to production.
Assume you made a program in DEV that uses another program which is a local object.
Obviously testing in DEV will be fine and your results will be fine.
But when you move to PRD, the local object won’t move, so the program will fail in PRD.
So, it is important to test in QAS.