1. You can read faster than a good portion of people can speak.
2. No one gets to know what you are conversing over. (teens especially love this)
3. No one has to hear about what you are discussing. (I'm talking to you Mr. really loud cell phone talker guy)
4. If you have a crappy memory it is there for retrieval instead of trying to recall what was discussed.
I hate texting but I ask my significant other to text me the grocery list so other people at work/bus/train don't have to hear/know that I need to pick up some rich chocolaty ovaltine.
Any given program will expand to fill available memory.