For example if they have kids and one of the parent stops working to raise the kids, divorce will make sure that person is not left without anything.
There is another civil union (the PACS) and as far as I know the main (and maybe only) differences is that PACS do not get divorce (you can part much more quickly, each party just get their own stuff and that's it) and something related to ineritance taxes. So that's the main reasons why people marry I guess :)
To be fair, I'll add that it's also not legal to marry religiously if you are not married civilly, so some people probably get married civilly just to be able to marry religiously.
It isn't the divorce which creates those rights, it is the marriage. The divorce rules only make it so that those rights are not abruptly suppressed after the dissolution of the marriage.
If you are married for some years in a state without common law marriage and then get divorced, you are probably due some of the house value, for example, even if it was in your husband's name. And one of the cars, and so on.
The same couple, not married, might have to sue to get one of the two cars and might not get any value from the house, even though they helped save for the down payment.
And there are lots of little things like that.
...what? Are you saying French people intentionally get married in order to reap benefits of divorce?