1. so incredibly convenient to always have with you, that everyone's willing to overlook shortcomings compared to dedicated equipment. Prime example: camera.
2. Offer a new type of use that is widely considered desirable. Example: mobile access to Internet.
Most use cases either cater to too few people and/or fall into the category "those who'd really care, already have dedicated equipment which is better". With flagship phones costing more and more, that equipment is probably also cheaper - or, at least, is price tag is not that outlandish.
Even replacing laptops probably won't catch on, simply because companies can easily provide a good laptop for half the price of a flagship phone (if not less). So they're not going to facilitate that. And if the boss isn't on board, would you want to use your own private phone as your primary work laptop?