Humans have a strong need to feel special, or unique, for more then just having abstract thought, which is really the only thing that separates us from other animals. They have a certain feeling of self entitlement, a desire to believe that they are so special that everything in the universe was created just for them. Some need a "crutch" to lean on or, when needed, to blame (ie. the devil) They have a whole in their psyche/life that some fill with drugs or alcohol or a god, they cannot summon the inner strength on their own (or so they think) so they use a god to help give them this hope to hold onto and get through life. But above all else, people need to believe that they and their loved ones will never cease to exist in some form or another. This need is so strong even Buddhism, Taoism, and Shintoism, which have no god or gods, believe that life doesn't ever end. It's hard for many to accept the reality of the universe and life in it and as has been shown throughout history we learn strongly toward being lazy, and the belief in a god makes things much easier to deal with and too understand.
two important reasons are hope and commitment. it helps us and makes sure we are able to commit to something and also it gives us something to turn to when we are falling apart and looking for some hope or help. :)
We don't actually need "religion" at all. God however, has created a need in us (from creation) to need Him and be in relationship with him. We are called to relate to God today through Jesus. When we die, we are told that we will face judgement, and that the only hope for us on Judgement Day is to be in relationship with Jesus. So in a sense, we don't need religion, we just need a relationship with God through Jesus. We also do need the congregation of other believers to assist us in our walk with Jesus through life (so this is where societies definition of "religion" can come into the argument) .Essentially though it is where we stand with Jesus that counts.
Some time ago there was a very popular song lyric which said, "People who need people are the luckiest people in the world." I think they had it wrong. People who LIKE people are certainly lucky, but people who NEED people are just a tad pathetic.
Now lets answer your question in that light.
We, as humans, don't NEED religion, but many people find comfort in it when things go awry in their lives, or when faced with personal tragedy. Not everybody can face the idea of oblivion with equanimity and for them, religion allows hope. If you were to take away that hope many would find life itself to be without meaning.
So religion fills a need for some but by no means for all.