Why add flakes sea salt on top of cookies?

It's a personal preference. I like to do it to some of my baked goods because the little bit of salt seems to round out the sweetness. Otherwise, it's just straight-out sweet and most times it's too much.

I make many different baked goods and I find it really enhances some baked goods by adding another layer of flavor while enhancing the others. I wouldn't call it a fad, its salt....I think people are realizing that the more we move away from ultra-high sodium packed processed food, that its ok to use salt more liberally, yet strategically, in the home kitchen. I find that if you salt baked goods in layers, you have a more dynamic flavor profile. If I'm making chocolate chip cookies, I'll lightly salt the dough and then lightly salt the top with something a bit more complex (like a smoked salt or maldon). It is perceived differently by your tongue than if you just put all the salt in the cookie. I could just make Toll House cookies and sell them.....but people pay me for something they can't get at home.

At least in my neighborhood (SF Bay Area), this fad peaked about 5-6 years ago and quickly faded; I don’t know about adoption in other parts of the country.

I have traveled abroad during that time and noted that the fad was not adopted at all elsewhere.

If you are a baker, it is very easy to sprinkle sea salt flakes on one or two cookies in a batch to judge how well it works.

It’s a taste preference thing. Do as you wish.

Best of luck.

It's just a taste preference. The flakes of salt add a hit of saltiness that goes well with some flavors, if you're into it. The flakes dissolve quickly and don't leave a lingering salt taste in your mouth, so the whole cookie doesn't taste too salty. Plus there shouldn't be much sprinkled on. Personally, I like the flaky salt on browned butter chocolate chip cookies where a little salt amps up the nutty butter tones and goes well with the chocolate. I wouldn't care for salt on, say, a jam thumbprint cookie. But that's just me. The best way to figure out if you like salt on cookies is simply to try it yourself: buy one next time you pass a bakery that sprinkles salt on their cookies and see if you like it.

It's a fad. As far as I'm concerned, it ruins a lot of perfectly good cookies and will eventually go away, but currently people are salting desserts and sweetening savories pretty indiscriminately.