Apple phone replacement

I had my iPhone 13 mini glitch the other day where I was on FaceTime and the screen turned to a static green screen and it has stayed that way ever since.

I went to the Apple Store and they couldn’t figure out what was wrong with it but said I could replace my phone for another mini (which is prob refurbished right as it didn’t come with a case) for $399 to which I agreed. 

I picked it up from the Apple Store today and the guy helping me asked if I had the old phone that was glitching. I said yes, do I have to return it (in the email they sent to pick up the replacement it mentioned to bring my old phone) and he said yes to which I just said ok. 

But now that I’m thinking about it, why did I have to return the old phone if I paid for the new one? It’s not like I got any trade-in value. Could I have declined to return it? Has this happened to anyone else?

Thanks in advance