Well, I don't think that prayer is always about selfish reasons. I often see people asking prayer for other people and not for themselves.
Prayer is about communication with God.
Just to give you an idea: When I pray, I talk with God, not only about my own needs, but also about issues with other people. I ask for wisdom on how to deal with certain issues a lot of the time. I also pray for people who are sick for healing. I also pray to worship God and thank him for the things he does in my life.
As to the selfish reasons, when my mom was sick, I asked for prayer and mentioned it on this website. Amazingly a few minutes after I posted the prayer request on here, my mom came out of her coma-like state. Was that selfish of me to ask for intervention on my mom's behalf?
There are scriptural references to "where to or three are gathered in his name, he will be there" and I have found that the strength of multiple prayers focused on the same issue does get God's attention.
To use an analogy, did you ever see the movie "Bruce Almighty" and see the volume of prayers uttered at one time. That's a good example of just how many prayers are going on. So, I think that it helps to have many people praying at the same time for the same issue, as there are so many prayers competing for God's attention.