Phones are for talking, navigating, and casual content consumption. Desktops (and laptops) are for actually getting things done. Both are useful, but the former is not a substitute for the latter.
Tablets are oversized phones that can't even phone. I don't see any use for them that isn't better served by something else. They'd actually be useful if they ran a desktop operating system, and some early ones did, but modern ones don't.