For us it was more of a why wouldn't we get married. There are sometimes good reasons not to, for example if you know what the legal and financial effects of marriage are and don't want them, but that wasn't the case for us. Why would cohabitation be better than marriage for us, and the answer is that it wouldn't have been. We wanted that legal, can't unilaterally withdraw it without the other one knowing level of commitment. It was emotional and practical, really.
It was a good decision and I don't regret it, as far as I'm aware neither does DH! Part of the reason it was a good decision is that, with respect to comments about people separating etc, married couples are more likely to stay together and are also more likely to experience the end of their relationship by death than separation. And marriage provides more benefits on death.
Now obviously the data about married couples being less likely to separate might be a chicken and egg thing, maybe it's being more likely to get married that makes you more likely to split, not the marriage itself. But at minimum, it's a refutation to the idea that it's less risky not to marry.
The only way to protect yourself from the possibility of a nasty, expensive separation is to not have a relationship in the first place. Or at least not one involving kids and/or property, since a twat ex can use the legal system to cause you shitloads of trouble relating to either of those, married or not.
In your own specific case OP, it sounds like marriage wouldn't change anything emotionally for you. For some people it doesn't. In that case, consider the legal and financial implications. That's easier in many ways, as it's not affected by anyone's feelings.
And as I said upthread, you might be assisted in making your mind up by also thinking of continuing to cohabit as an active choice. So not just why marry, but also why cohabit. Get yourself thinking about what the positives and negatives of both would be for you. Makes sleepwalking into a situation less likely.