Yeah, like you said, it's mainly for text layout purposes I'd say. It's much easier to format text/paragraphs under the assumption it will always be a fixed width and will display the same for everyone.
The forums itself here though are flexible (percentage based) which means they expand with your browser window. Although yes it does "fit" the screen, reading text is rather annoying I find. It means something that should be a few lines long ends up being a single line stretching across my 2560px. Eye fatigue becomes quite an issue for me
Usually I end up keeping my browser window to half the screen when I'm actively reading. Then it's more natural feeling.
It really depends on the site and content. Obviously Twitter has no reason to be any wider because the messages are limited to 150 characters? There's no reason to make it stretch out.