We already have a "space force" - the Air Force - which was spun out of the Army. It's not odd to spin out the Space Force from the Air Force, if necessary.
> And why we need any kind of weapons in space?
Because other nations want to or are in the process of weaponizing space.
The first time someone knocks out one of our GPS satellites, you'll be asking, "why don't we have a Space Force to protect against this sort of thing?"
Everyone's favorite rational pop scientist, Neil deGrasse Tyson, thinks it's "not a weird idea." [0]