Because [Hela and Surtur are] trapped in the lower, intra-dimensions of string theory, at the base of the world tree. They have to come through Midgard and Jane Foster... Then Darcy becomes Hela for a while leading into the next films.
. . .
Hela needs Jane Foster & Malekith to break her and Surtur into the higher dimensions. Because Odin has already trapped them in the inner realms for all existence.
It's from the inside that hell begins to break loose and Malekith intends to raise it.
(I missed this earlier; it looks like you may have added it later.)
Wow, that sound pretty fearsome for both Jane and Darcy! I guess this would be a case of being careful of what you wish for (if this is how it plays out). Jane is certainly relevant to the story in a way that goes beyond being a love interest (and connected to her work). And Darcy has a larger role.
(As I observed earlier, this is an unusual treatment for one of the good guys, but not unprecedented. And I'm still not 100% thrilled, but I'll run with it here.)
But why would those guys need Jane and Darcy? They're so powerful on their own. And if they do possess J&D, why would they need to do so for more than 10 seconds, long enough to get all the knowledge they need? And while opening a portal is a mean feat for a Midgardian, by your outline the people of Midgard are not all that powerful. So why would it be useful to possess Jane and Darcy?