So you can communicate with someone literally all day. Talking to someone over the phone is very personal and sometimes intrusive. With texting you can communicate casually. To call someone you usually have to have a point, the conversation ends, or they end because someone has to do something.
Anyone who has had an extended conversation with a girl over text message would know what I'm talking about. My friend for example, text his girlfriend literally every 3 minutes of the day, unless something like school gets in the way. If taking a shower he would simply stick his hand out the shower to text, he has someone text for him if driving, while playing COD he simply drops the controller and picks up the phone to respond.
If it weren't for girls, there would be no need for unlimited texting.
Personally, I put alot of thought when I text girls so my response time isn't even close to his, but I'm working on it. And that's another thing, you could be terrible at communicating your feelings in an instance but a fucking wizard at texting.