When you open it, it becomes a tablet not a phone. And therein lays your answer.

Turns into a small tablet which is great for reading, for impromptu client demos, and for playing some games like puzzles.

It also enables multitasking by having applications side by side which can be very useful on a phone too.

You don't have to use it all the time, but it's nice when you need it.

What a strange question. I guess we no longer need monitors because a phone display is all anyone should need? Why do I need two devices when one would do? I get we are not there yet but I can certainly imagine having just one device and virtual displays in my glasses / contacts. Those displays will not be phone sized

Also, bad eye site, large fonts, suck in small screens

Makes it easier to use enterprisey management console type UIs on the go. Also nice for reading large diffs.

I actually develop software on my phone with Termux. I want the biggest screen that still fits in my pocket.