You might like this CGP Grey video that explains it pretty well:
The Better Boarding Method Airlines Won't Use https://www.youtube.com/watch?v=oAHbLRjF0vo
Every time I accidentally fly Southwest, I'm horrified at how bad the boarding system is. People plop down wherever leaving one-seat holes so you can't sit with your partner.
It also guarantees more seat trading than an assigned system because most people in the assigned system have seats they wanted. People on Southwest flights are always haggling so they can sit with their partner. I almost never see seat trading on other flights.
My mind is blown. But then again every time I take a Southwest flight, I'm wondering "who tf is this for?" and I've finally found my guy.
The times I didn't check in early could suck, but I was pretty good at spotting couples that had split aisle-window (leaving the middle seat empty), and then calling their bluff.
But it's been several years since I've flown, and I'm sure all carriers have since innovated new ways of making the experience worse, while charging for the privilege of mitigating it.
You can still pay more for earlier boarding - families with young children board first
Other airlines will gladly assign you a seat for a price
Most airlines’ boarding strategies are terrible, but getting rid of assigned seats would objectively make my flight experience more chaotic.
And because corralling children (even teenage children!) is much harder than just getting up and walking on yourself, this results in families almost invariably having to either accept being spread across separate seats in multiple rows, or being pushy about trading seats with exactly the kinds of people who have aggressively made sure they get there first.
Still it would have been nice to see a comparison of the times between non-assigned front-to-back boarding and traditional assigned boarding. But I can imagine that's not so amenable to simulation.