Yeah I hate that. There's no way you can't communicate what you need over email or IM. And if you genuinely can't, well, you have serious communication issues.
Writing the same content as an email takes me like 10x the time. Especially if it's something complicated and people might have follow-up questions so it takes up room in the back of your mind. Much easier to just get out of the way in a 5 minute conversation.
Anything to do with emotions or interpersonal relations is also very prone to misinterpretation when put as text.
A lot of people really can't communicate, or just aren't sure what they want in the first place.
So they can't explain over the phone either, but fifteen minutes of vague rambling lets them feel like they explained it, and there's no written record of what they did or didn't say so they can blame you if you can't work it out. It's win/win, as in they win and blame you regardless of what you do.