I'm not religious myself, so I can only state what I believe may be reasons why others are religious based on my observations. I'm sure that not everyone will agree.
1) The fear of death.
2) To lessen feelings of helplessness.
3) The hope that people more powerful than themselves will be held accountable for perceived wrongs in an afterlife.
4) The need for future rewards to help them make moral choices today.
5) Indoctrination from parents or other authority figures.
6) The unwillingness to believe that the universe will continue just fine even though they no longer exist in any way.
7) The thought that heaven will provide them with the feelings of satisfaction, peace, and justice that they haven't been able to find in life.
8) The desire to see loved ones made whole again.
9) The desire to get answers to questions that really have no answers.
10) For a sense of community with like-minded people, hopefully for the purpose of doing something that they feel is helpful to the community.