I have no idea why Hashicorp didn't use it. The point is just that if you made a contract for something and they hold up their end, you've gotta hold up your end, even if you ended up not using what they provided.
I'm sure there are cases where doing a specific thing (e.g. buying another company, or paying off particular debts) with the money is part of the deal. And sometimes an investor will demand a board seat which gives them a way to influence other decisions. But I'd be amazed if a VC wanted a "spend the money on something" provision - if you really can't think of anything to use the money for, letting it sit is better than burning it. You're always obliged to use your best business judgement on behalf of your shareholders, which would seem to cover making use of the money if you have a good use for it, and not if you don't.
Yes. Just like you still pay interest on a loan even if you just let the cash all sit in a bank account.