What is the mentality of texting each other?

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From what I saw, my little sister loves to text with her friends, and she can text it all day without getting her fingers sore. Luckily, we sign up the unlimited text plan, otherwise, it will cost a fortune to pay for all these unimportant messages. I would rather talk over the phone, instead of spending so much time on texting. What is the actual mentality of texting? Why not talk straight out?