A ceremony is not necessary, just a way of sharing your happiness with family and friends. The ceremony need not be religious.
A ceremony is a very nice way to participate and enjoy two friends officially getting together; I think it adds a bit of slapsh and nothing to do with religion... But if people think mutual love is enough, by all means it's OK too.
Oh! It is possible to be wed without all the trimmings of a social get together to celebrate the wedding.
As Neal pointed out it is a way of sharing your happiness with family and friends. Because it is also life changing event where no matter the style of ceremony you choose, you no longer are single where you really only had to think of yourself and for yourself but you now have a partner who must also must be taken in consideration in your decisions.
A wedding ceremony is as old as humanity you will find.
Usually, you don't wed just a person. You (and possibly your kin) also connect with that person's family and friends. By no means an essential condition, but in most cases it's how it goes. Depending on the importance you and your mate give to these connections, they might want you to have a ceremony that includes them.