While I pretty much only go watchless when I forget my watch at home, I do like the idea of going watchless on easy runs for a lot of people. The reason is because some people become a slave to the watch. If you're one who checks splits and either consciously or subconsciously adjusts pace to hit some arbitrary number, then you will likely benefit by going watchless and just not worrying. If you don't look at your watch but are concerned about what your watch will say at the end and that affects the pace you run, you will also likely benefit.
In short, the benefit of going watchless is that you don't become a slave to the watch. You can run whatever pace you need and not worry about hitting some arbitrary number.
Of course, this benefit has two sides where it could potentially make no difference. First, if you can run with a watch but not worry about what it says, then there might even be a benefit to wearing the watch. I look at the watch as a tracking tool. When I'm running, I don't care what it says. After the run, I'm not worried about hitting some arbitrary time. I just look at the time as an indication of how things went. It's another piece of feedback that I can factor into how the run went after the fact, not some target to hit. Second, some people will still try to race the clock even when they don't know what the clock says.
To truly experience the freedom of running without a watch, you have to have that watch free mentality of not caring about your pace. Just go at an effort level that feels right and let the pace take care of itself. Of course, this is possible to do (or to not do) with or without a watch.