Beside, you know... storing stuff.
I used it to give my Hunter a makeover; I took all the Legendary and Exotic gear I had on her (plus everything else in my inventory; strange coins, commendations, ether seeds, heavy ammo, you name it...) dumped it in the Vault, deleted my Hunter (a Human Female) and made a new Hunter (an Awoken Female).
Since I used to have 2 human characters and 1 Exo character, and my Hunter looked a bit ugly, I thought I'd take advantage of the extra Vault space to hold my gear so I could make a new Hunter.
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Filled it instantly. Already wanting a larger vault but with the weapons in subcategories.. Pulse/hand cannon/AR/scout ect.