Just curious: we are considering enabling this feature. We have Contacts that are going to be related to multiple Accounts. Which one will Salesforce pick as the primary Account?
Edit: reading the given answers, I realized that I had made a thinking error. In our org, we have several Contacts that should be a single Contact, but aren't. They share things like name and email address. But right now, each of these Contacts has the Account to which it belongs as its primary Account.
In the future, we must merge these Contacts to get single Contacts and while doing that, we must decide which Account will be the primary one. When enabling the 'Contacts with multiple Accounts' feature, Salesforce will not do any merging, so it doesn't have to decide which Account will be the primary one.