Why do you need to enter time slot to purchase a ticket. What happens if you arrive late or decide you want to come earlier?

It's just a fallback i think. It really doesnt matter when you walk in or out.

It's time frame to insure enough space and quite time for you to enjoy visit

You can arrive later than your time slot, but not earlier

Its best just to buy your tickets when you arrive at the garden, so you don't end up being late or too early to go in