Some venue operate a guestlist to help control the numbers of guests, giving managers a good indication of whether the night is going to be busy or not and if they are going to need extra stock, staff or security.
As for event attendees; getting on the guestlist has its advantages as it improves your chances of getting in without a long wait, but it’s not a guarantee. Furthermore most venues will offer some form of reward for arriving early. Typical rewards can include things like cheaper entry.
So its in your best interest to join one if and when available!