The "new Space Force" is just a renaming of existing assets (https://en.wikipedia.org/wiki/Air_Force_Space_Command). Hell, it's still under the Department of the Air Force and its Secretary.
My guess is that it's purpose is ABM.
As with the staffing, that's largely just a transfer from the Air Force.
https://spacepolicyonline.com/news/first-budget-request-for-...
> Most of the funding for Space Force in the F2021 request simply reflects shuffling funds into the new account structure, but an Air Force official said today there is a $900 million increase compared to FY2020.
Fighting such a war without a unified command over the theater in which that action unfolds seems destined for failure.
And failure to preserve or triumph in space substantially degrades planetary military capabilities.
So really, Space Force is an expression of "space is important to all the other military things we do now."
NASA is a civilian agency. It isn't and never has been justified by anything related to military defense.