Why the time gate?
Content in MMOs is frequently timegated to keep people from rushing through content and then not having anything to do so they leave the game.
ANet may give it to you.
As JAFW said it’s to stop people rushing through it. It means you have something to do for longer, and you’re going back to the map regularly – things like that spread players out and create more activity in the game as a whole.
Unless you’ve already finished everything else you need to do for this tier you can use the time to work on other parts of the collection. And if you have finished everything else I recommend you use the wiki to look at what you’ll need to do next and see if there’s anything you can get started on now, even if it’s just collecting raw materials.
Legendaries are designed to be long-term goals. A lot of the process is very time consuming, either because it’s time-gated or because it needs a lot of materials, expensive items or achievements which take a lot of time and multiple attempts to get. It’s a good idea to plan ahead and start on the stuff which is either time-gated or simply going to take ages first, so you don’t finish everything else and find yourself hitting a road block with 1 item.
“Life’s a journey, not a destination.”