Someone, somewhen, had that bright idea of popularizing 16:9 and 16:10 screens, which are now de-facto standard, and that leads to huge amounts of screen estate being wasted. Most of the users (i.e. those not using tiling window managers on Linux systems) don't have proper tools to manage screen space, and especially web browsers when maximized waste most of it.
So TLDR; yeah, I could use a messenger tab side by side with rest of the browser tabs, because I have a shit ton of useless screen space with a maximized browser window.