It's useful if your integration work takes some time - easy to run into with open source.

Imagine you have multiple contributors with multiple new features, and you want to do a big release with all of them. You sit down a weekend and merge in your own feature branch, and then tell everyone else to do so too - but it's a hobby project, the other guys aren't consistently available, maybe they need two weekends to integrate and test when they're merging their work with everyone else's, and they don't have time during the weekdays.

So, the dev branch sits there for 2-3 weeks gradually acquiring features (and people testing integration too, hopefully, with any fixes that emerge from that). But then you discover a bug in the currently live version, either from people using it or even from the integration work, and you want that fix live during the week (specific example: there's a rare but consistent CTD in a game mod, you do not want to leave that in for several weeks). Well, if you have a branch reflecting the live status you can put your hotfix there, do a release, and merge the hotfix into dev right away.

Speaking of game mods, that also gives you a situation where you have a hard dependency on another project - if they do a release in between your mods releases, you might need to drop a compat hotfix ASAP, and you want a reflection of the live code where you can do that, knowing you will always have a branch that works with the latest version of the game. If your main branch has multiple people's work on it, in progress, that differs from what's actually released, you're going to get a mess.

And sure you could do just feature branches and merge feature branches one by one into each other, and then into main so you never have code-under-integration in a centralized place but... why not just designate a branch to be the place to do integration work?

You could also merge features one by one into main branch but again, imagine the mod case, if the main code needs X update for compatibility with a game update, why do that update for every feature branch, and expect every contributor to do that work? Much better to merge a feature in when the feature is done, and if you're waiting on other features centralize the work to keep in step with main (and the dependency) in one place. Especially relevant if your feature contributors are volunteers who probably wouldn't have the time to keep up with changes if it takes a few weeks before they can merge in their code.