In modern browsers, whether or not lang is provided, the browser is smart enough to set the correct encoding / glyphs.

In the olden times, browser was not smart (ie: still set the wrong charset). You may have come across for example, an asian site (japanese, korean, chinese), and see a lot of text with "?"'s and "□"'s like so ??? □□□ in 90's and common still through beginning 2000's. Glad these days you don't have to spend time trying to match it up, but it was available readily in browser's menu for you to match.

I think here, the lang tag is useful so you can explicitly tell the browser. And if you are designing the page, you have more control for localization to target by the international code.