When I write an email, I'm usually (mostly) doing that one thing, and the result is quite joined-up. Anyone who has had the experience of trying to carry on a conversation with me over messaging when I'm doing multiple things knows that I can easily lose the thread of an explanation and I often find I have to read back the logs of a conversation to be able to keep up.
When someone takes the time to write an email, the reader can't interrupt or divert the writer and so the writer is able to express their points at their own pace. Email is also easier to re-read and reformat, if you are the kind of writer that doesn't necessarily think of everything that needs to be said in quite the order it ought to be said.
When someone is emailing asking me to do something, I won't lose an email (you'd be surprised how often I lose things people ask me to do on IRC, its on a par with asking me in the pub). Admittedly, my inbox can be several months deep but when I have time to attend to your query, I will - and that goes for social correspondence as well.
Finally, and most importantly, it enables me to stay in touch with people whose committments don't fit around mine in terms of online availability due to work, life, or timezones.
I'm quite enjoying a slightly more formal exchange of the written word with those friends who are emailing me while I'm on these little jaunts - you know who you are and I don't enjoy being away from home so those little messages are welcome, thanks :) And for everyone else - next time you are lurking online looking out for someone, take the time to write an email instead - and let me know how it works for you.