Deaf people are an obvious example. As are people dealing with content not in their native language. Accents can also be an issue.

And then there are many people who would simply prefer to read something over heating it. Is that really so hard to understand?

I can read faster than most people talk. With text I can go back and re-read the bits I'm having trouble understanding and skim over the stuff I already know. With text the is never a problem with hard to understand accents or people who mumble or speak softly. In fact, thinking about it, text is almost always better to deal with than spoken language.

Why it would be?

I sometimes have trouble conversing in crowded places if there's lot of other people having their conversations loudly. Not much trouble, though, but listening to the only one thread of discussion requires bit extra mental effort.

Then there are people who don't speak clearly, and you have to ask them to repeat what they said...

And all this in my native language. I find dealing with written forms of any foreign language I know much easier than the spoken language. (Both "understand" and "produce by myself".)