Both parties are computing some spreadsheet formulas contributing their own secret inputs, in such a way that both learn the result but not the input of the oth

Both parties are computing some spreadsheet formulas contributing their own secret inputs, in such a way that both learn the result but not the input of the other party: it's cryptographically impossible to do so. Regular collaborative spreadsheet software doesn't do that: actually, this is the first program for normal computer users that is able to do so (and not cryptographers specialized in secure multi-party computation). It depends on your business: if you are bank, a financial firm or any other that handles very valuable and confidential data, you need secure computation on any of its forms (garbled circuits, secret sharing, homomorphic encryption). Encrypting and emailing is encryption at rest, you're not encrypting the computation.