Why Force H needed fast capital ships

Why were battlecruisers assigned to Force H, and would they still be needed in the Mediterranean?

Their speed let Force H cover Italian threats and Atlantic raiders at once; a loyal French fleet reduces that dual burden.

Renown and Hood were valuable because speed let Force H cover both the western Mediterranean and the adjacent Atlantic approaches. Losing the French fleet removed strength in the Mediterranean and French coverage against German surface raiders. Gibraltar therefore became the hinge between two operational problems. If the French remain Allied, Britain can distribute scarce fast capital ships according to other Atlantic and global demands.