My guess is that they rent vehicles for undercover operations to better disguise themselves.

Perhaps a rented vehicle is more normie and blends in better than a full-time government vehicle.

Secret Service probably has to blend in with crowds, vehicles, etc. in order to protect the pedophile criminal politicians.

I guess when a President is in a large stadium, there's probably secret service agents embedded in all the sections of the stadium with duty to monitor that assigned area.