To give the mentally weak a sense of relief, to give the ones who are afraid of the dark something to hold on to, to give rulers something to control mass public with, to brainwash citizens from the truth, to cause war, prejudice and hatred, to pull people and countries apart.
The potluck dinner. Srsly. I've noticed that most religions have something like this. Christians with the bring yo' jello and cream of mushroom soup recipes, Hare Krishnas and Sikhs sharing vegetarian meals with the public...
It varies from region to region. In America, it seems like its only purpose is money. In the middle east, it seems like it is used for brainwashing and control. In Europe, it really doesn't have a use and is starting to fall apart.
That's a really broad question. On a personal level for different people religion might serve as a source of comfort. Perhaps it gives them a sense of belonging and all that.
In other contexts? Karl Marx famously said that "religion is the opium of the people". Is religion perhaps a tool invented by authority figures to control the masses?
Many of these Evangelical Christians on television you see use religion as a tool to make money off their followers. In that sense, religion is just a business.