On a weekday there is usually a room free, and walk-in guests are not treated badly. But the evening starts with nobody in charge of your table.
You get an assigned manager. With a booking, someone knows when you arrive and how many you are, and the same person looks after you from the room to the final bill. Walk in, and whoever is free takes you on, knowing nothing about your budget or plans.
You get a better room. Rooms go to bookings first. There is only one VVIP room (602), so getting it as a walk-in is mostly luck.
The 10% discount is for bookings only — on the room, drinks (soft drinks included), food and the service charge, never on tips.
At weekends there may be no room at all. Six people arriving unannounced on a Friday night have a lower chance of finding a room for six than they expect.
A booking isn't paperwork for the venue. It sets how you will be looked after that night, and it takes one message with the date, time and number of guests.