I am on holiday and have no mobile signal - but can use broadband (I can't use the associated landline for calls)
I need to exchange occasional messages with a friend who is currently abroad and so has mobile signal but no internet (because of ludicrous roaming costs)
Skype could do for calls (but would be quite expensive for both of us, I would think), so I'd rather stick to messages (it's also the communication method that suits us best anyway).