Sure, in an ideal situation people would naturally mingle, but a lot of people are shy or will just stick to people they know, which makes it less valuable as a social event.

As someone with a medium amount of social anxiety: for the love of god, arrange for excuses for me to talk to people. I cannot just walk up to people and start a conversation. If your party does not have reasons to talk to people I will not and just end up feeling like a third wheel.

I'm sure there's plenty of people who are naturals at parties, but for those who are not it's very nice if the deck gets stacked in favor of new interactions.