"What I don't understand is - surely raising your child(ren) together is the ultimate commitment. So for people who marry say five years after having two children - does it nevertheless signify a new step in the relationship? Or is it merely a delayed celebration of the fact that you are already de facto married?"
It seems to me that you see marriage as a public announcement of commitment and nothing more. It is the 'more' that matters.
You could say that marriage doesn't matter - when everything is going swimmingly. It is when things are going badly that it becomes important. Because marriage is NOT just a public announcement, it is a legally binding contract, one that confers rights and responsibilities on those party to that contract.
Now, the 'going badly' comes in many forms. One of you dies. Married, you are their next of kin, with the right to arrange their funeral, inherit their goods etc. Unmarried, their parents are the next of kin and they are in charge of what happens next. Including, possibly, whether you get to stay in your home.
Or the relationship goes south. You won't be surprised that again, rights and responsibilities are legally binding. Particularly important where children are involved.
Oh, and rid yourself of the notion of "already de facto married". No such thing, even though many will wail that they are a "common law spouse". Marriage is a legal contract, if the contract has not been made then there are no rights and no responsibilities on either party.
Many think of a wedding as a big party. It's not - it's just a party tends to be held to celebrate the contract. It's not 'romantic' to regard it as a legal contract; but I've never found the mess that can be left by not entering into that contract as particularly 'romantic' either.