Well, lets extend your reasoning a bit further.

Why do we post on a forum? This is pretty much all frivolous squabbling anyway.

Why do we talk to our friends. Most likely the conversations we have with them will be moot 5 years down the line.

Why do we continue to live. It seems like a complete waste of energy. In the cosmic scheme of things we're expendable.

The simple, and only reasonable explanation I can think of to any of these questions is because we want to. It makes us feel better to enter a clean room. It occupies our mind when we create a Lego fort. Text messaging also serves a variety of purpose. For instance, you may not feel like having a personal conversation. Text-messaging is a lot more impersonal than talking to a friend. We do all of these because we prefer to do so. We decide what we want to do. We assign the purpose in our life. That's all there is to it. Those two basic premises must be set before we can even attempt to derive any rationality behind our actions.

And we pass on these seemingly pointless habits to our kids because we feel that it will help them cope as it did for us. Perhaps it is a flawed logic. But I am certain that that is why we do it.