But yes, of course a phone client does not preclude a desktop client; it's just that being phone-first forces/nudges people to having it on their phone as well, making it more valuable for everyone they know.
The point of the laptop is not checking the messages, but typing them. If you're at home, which is where a lot of us are these days, it's pretty easy to open the laptop and save time on typing.
BTW, not sure why I got downvoted. Maybe it wasn't you, we'll see.
My point was exactly that pressuring people to have the client on their phone makes them check the messages more often, because that makes the service more attractive to other users. That, I think, is the reason most popular service are phone-first, even when typing messages is easier on desktop.
(And no, I didn't downvote.)