The fact that Zcash is versatile in that way will allow for innovative wallet integrations where someone simply has a private/public button to click before sending a transaction. There are also many reasons people, organizations and companies would want to send money in private and they are perfectly legal. While there is a huge focus on the darknet use of this technology, something like sending money to Wikileaks or hiding taxable money from a government may also be interesting and valuable.
Even simply paying employees via blockchain without anyone knowing how much you’re paying is a big deal and totally legal.