Any religious, social or symbolic connections aside (as any value you may or may not place on these aspects is subjective), the legal benefits alone are pretty vital to most adult couples.
To name just a few:
-Next of kin status. If the person you care most about in the world is seriously ill, that marriage certificate can be the difference between being able to be with them and being made to wait outside.
-Inheritance. Let's be honest here- who's actually got all of their affairs in order and written a Will? Not many of us, even though we all know we could get hit by a bus and die today. You can find yourself looking at a tangled web of legalities and complications in the event of the death of your partner as it is, being legally married before their death gives you both considerably more protection.
-Increased rights for fathers, should you have any children together. The law in many states and countries doesn't officially recognise a man to be the father of a child unless he is either married to the mother or they have a paternity test. For example, here in the UK, a father can not register the birth of his child without the mother present, unless they are married. There are also more serious consequences, such as a man that was once married to the mother having an increased likelihood of being granted better visitation rights or co-parenting status, should the couple separate.
-Shared healthcare coverage. Pretty sure that's a big one for a lot of you folks in the US- many company-provided healthcare plans also offer coverage for your partner... but sometimes this is only on the condition that you're legally married. I've heard of multiple cases of an absence of a marriage certificate causing couples serious problems when trying to claim off their partner's insurance.
-Immigration rights. This was an important one for my husband and I- due to the nature of our careers, there is always a chance of one of us being offered a job opportunity abroad. Because we're married, we can immigrate together. Without being legally married, it'd be tough s**t- you'd just have to choose between your career and your relationship
There are more, but this post is turning into an essay already!