Why do restaurants use temp staffing apps instead of just hiring you directly?

I’m genuinely confused and would love to understand how this works.
I recently had an interview at a restaurant and it went really well. I told them I do temp/extra shifts, and everything seemed good. Today, the manager called me to say the interview went well and asked if I could start Monday. Obviously, I said yes.
Then she told me she would post the shift on a temp staffing app, and I would have to apply through the app so she could accept me.
I’m just wondering: why can’t they simply give me a temporary contract directly?
I understand that these apps probably make things easier for the restaurant when it comes to paperwork, payroll, etc. But if they’ve already interviewed me, chosen me, and want me to start Monday, it feels a little strange to have to go through another platform just to officially work the shift.
Is this mainly an administrative thing for the employer? Or is there another reason restaurants prefer these apps?

I’m not trying to avoid doing things legally or anything. I’m just genuinely confused about how this works.