a) Knows how to use email
b) Has access to email
Don't underestimate these things. If I'm at home and I need to log into a company VPN just to update my status for my manager, that's a lot more effort than sending off a quick update via email or just picking up the phone.
Conversely, if someone asks me for some help on a project and then asks me to learn an entirely new system for an hour of my time (including new logins, new bookmarks, possibly installers, etc) it is often a net loss for me and for them - by the time I am up-to-speed I am done with what they needed anyway.
Basically, I am happy to use whatever system can help me do my job better, but too often I am forced to drop back into email anyway. Eventually I am trained to just use email first, because once your tool gets out of sync, its usefulness decreases.