I don't know if someone replied already to you or not (it doesn't seems so to me...) but piecing together all the Thanos logs which happens right before the Monopole reveal, how Thanos says it's mainly a deterrent, how it's encased in a protective field, and how he said constantly to Jared that he hopes he can "wield his arrow" heh... my guess it's:
A fucking mass-destruction weapon. Either an "arrow" that can breach the 1st-2nd spheres, ensure complete obliteration inside a certain radius (or hey, maybe it's planetary scale or bigger!), or the arrow itself is made of/contains antimatter from the 2nd sphere. If Jared were to absorb it, it could be safely stored, moved, and deployed even with the bonus of Jared being a pussy in terms of power/evolution level but hosting something truly dangerous HERO would hardly pick up on until it's too late.
Also, I could easily see the deterrent not being "The Berhant arrow annihilator!" but rather something that could wipe out humankind lesser superhumans from Earth, ensuring its extinction regardless of if HERO member can tank the damage or not, hence another reason for a Mars base that can also move around.