A URL, for instance, can't be safely stored in 768 characters, but it can be stored safely in 2704. If you then wanted to sort those URLs so that all URLs for each domain and path within that domain are adjacent, you need an index. Especially if you want to paginate over them with a cursor. Doing that without an index on the raw value is a royal pain in the ass.

Hell, even just being able to sort user-submitted strings up to a kilobyte. Why up to a kilobyte? Some users have strings that are kind of long. If I have to define a second column that's the truncated prefix, that's just a silly waste of space because MySQL decided to use utf-32 under the hood.

You might just load someone else's data, and the index is desirable in general for speeding up analytic queries. It's possible to work around that, of course. But depending on what you do, it can make writing efficient queries against the data more difficult. That's just a distraction because most of the time, those long columns won't matter anyway.