As far as audio goes, either you need to play something in your headphones (which itself can be distracting) or sounds will go through (actually many will probably go through either way), and sometimes that's perfect for what you're working on, but sometimes it very much isn't. Also they're not an option when you're trying to talk to someone without both of you getting distracted or distracting others.
Headphones don't filter out every distraction either, though, so there's that too. People constantly walking around or eating food you can smell or shuffling stuff on their desks or doing whatever else they are doing can be incredibly distracting when you need to concentrate.
Also distractions are one thing, but I know lots of people want some privacy in the workplace too, even if it's not the first (or last) reason they'll list. It's kind of the elephant in the room. If you're the kind of person/robot who thinks privacy in the workplace is a fundamental oxymoron and who goes 4 or 9 hours straight without taking a bit of a break once in a while (whether it's just to check your personal email or social media, or to lay your head down for some time and rest a bit, or to make/receive a phone call, or whatever) then maybe you won't sympathize with this. But not feeling like people are continually observing you is something a lot of people value.
Also my ears can eventually sweat when I have headphones on for hours on end, and my headphones end up getting dirty, and their existence isn't exactly indistinguishable from their non-existence either, so there's that. Maybe this is too obvious? But an open-plan office where the solution is "wear a headphone" is practically a job that mandates that you wear a headphone.
I'm sure there's other stuff I'm not thinking of at the moment. And of course there are lots of benefits too; I just didn't list them since you seemed to already be aware of them.
Headphones don't filter out every distraction either, though, so there's that too. People constantly walking around or eating food you can smell or shuffling stuff on their desks or doing whatever else they are doing can be incredibly distracting when you need to concentrate.
Also distractions are one thing, but I know lots of people want some privacy in the workplace too, even if it's not the first (or last) reason they'll list. It's kind of the elephant in the room. If you're the kind of person/robot who thinks privacy in the workplace is a fundamental oxymoron and who goes 4 or 9 hours straight without taking a bit of a break once in a while (whether it's just to check your personal email or social media, or to lay your head down for some time and rest a bit, or to make/receive a phone call, or whatever) then maybe you won't sympathize with this. But not feeling like people are continually observing you is something a lot of people value.
Also my ears can eventually sweat when I have headphones on for hours on end, and my headphones end up getting dirty, and their existence isn't exactly indistinguishable from their non-existence either, so there's that. Maybe this is too obvious? But an open-plan office where the solution is "wear a headphone" is practically a job that mandates that you wear a headphone.
I'm sure there's other stuff I'm not thinking of at the moment. And of course there are lots of benefits too; I just didn't list them since you seemed to already be aware of them.