Mjolnir in the MCU is extremely crucial. In fact, it has got more to do with the films than War Machine has. The same goes for the hammer in the comics. And actually, things in the comics are slightly different.
In 2014, Thor became Unworthy of Mjolnir in Original Sin. A while after that, Jane Foster picks up the hammer & becomes the Goddess of Thunder. But back in 2018, Jane sacrificed Mjolnir into the heart of the sun.
Like in the MCU, it seemed that Mjolnir was gone for good. But it returned in Endgame & will be back for Thor 4 as well.
Similarly, in 2019’s War of the Realms, Thor brings back the hammer. He re-forged the hammer from inside the sun itself by using the God Tempest.
Since the hammer was originally forged from the heart of a dying star, it makes sense. (Infinity War) Mjolnir falls on Earth, and Thor once again is worthy of the hammer. Since it came from within the sun, it had no handle.
To reference the MCU, Thor makes a wooden handle. It doesn’t come from Groot though.
He used the branches of Yggdrasil, the metaphysical World Tree that binds all the Ten Realms. Just like the Stormbreaker in MCU, it also has a new power. This hammer carries the king’s word wherever it is sent. Thor once throws it through every single realm, and Mjolnir carries his announcement.
This could be used in future MCU movies as well. Now, that we have explored Time Travel and saw the hammer in Endgame. There is a lot of possibility we might see it coming up more in Disney+ series or future MCU movies. In Thor Love and Thunder, Jane Foster will also have the power of Thor, so we can surely see the hammer there.
But before the hammer gets back to him, it ends up on Jotunheim once. Here the King of the Frost
Giants, Loki picks it up. It seems that he has cast a new enchantment upon it. The hammer now seems to be getting heavier for Thor.
Perhaps it is getting lighter for Loki? We’ll find out soon.