Past test 2 days ago but they didn't keep my provisional, HELP

Help please

I had my driving test 6th march and passed (1st time), I passed my test but the examiner didn't keep my provisional licence, they asked me to fill in a D1 (?) form and send it to DVSA with my provisional licence and my pass certificate. I am unsure why I have to do this and why they didn't keep my provisional licence themselves.

Can I get any assistance in this?