Asking guests to scan creates a queue you cannot control
When guests scan a code themselves, everything depends on their phone, their network and their patience. People far from the poster struggle, some have no data, and a few simply declare they arrived without doing anything. You end up with a slow door and a list you cannot trust.
When your staff scan, the door moves at your speed
Guests only show their ticket, on paper or on a screen. Your colleague scans one after another, hears a sound for each accepted ticket, and reads the guest name on the phone. Nothing depends on the guest's phone working.
It also protects you from double entries
Because the scan happens on your device, the rule you chose is applied every time: a ticket already used is refused on the spot, in front of the person presenting it.