A restaurant waitlist app replaces the clipboard at the host stand. The host adds each walk-in party with a name, a phone number and a party size, the app gives a quoted wait, and the party is seated or marked as left when their turn comes. Better apps text the guest when the table is ready so nobody has to crowd the door.
Most standalone waitlist apps leave the quote to the host's gut. Reserving works it out. It looks for a table that fits the party right now. If none is free, it takes the earliest moment a fitting table clears, using the end time of the booking on it, then adds the parties of a similar size already waiting, and rounds up to five minutes.
That is why the quote holds on a Friday at 7:30. A four-top booked for 8:00 is not offered to a walk-in who would still be on dessert at 8:40.