Why would you place potatoes on salt when baking in the oven?

This is what America's Test Kitchen (sorry, paywalled) has to say about it:

Sometimes baked potatoes can use a flavor boost. And instead of light and fluffy, most often they are dense and crumbly. We found that baking the potatoes on a bed of salt remedied these problems. Moisture that escaped the potatoes during baking was trapped in the enclosed pan, absorbed by the salt, and eventually reabsorbed by the potatoes, making their skins tender and their flesh light and fluffy. All we needed to do was fine-tune the variables. Using a hot oven and uncovering the potatoes toward the end of cooking ensured dry, crisp skin. A 13 by 9-inch baking dish provided plenty of space so that we didn’t have to crowd the potatoes, and 2 1/2 cups of salt allowed us to thoroughly cover the bottom of the pan.

I have found that salt roasting potatoes makes the skin crispy, without it getting hard. Like the above says, the potato itself seems fluffier. The way the salt seasons the potato is particularly nice as well. Incidentally, you can reuse the salt over and over again, so it's not as wasteful as it might seem when you first consider the method. You'll want to keep separate salt for things like this, though, since it picks up potato flakes and discolors a bit.

EDIT The ATK method calls for covering the 9X13 baking dish (for 4 potatoes and 2 1/2 cups of salt) tightly with aluminum foil and roasting at 450F (232C) for 1 1/4 hours. Remove foil, brush potatoes with oil, raise oven temp to 500F (260C) and continue roasting for 10-20 minutes, until potatoes are tender when pierced with a knife and the skins are glossy.

There are many roasting recipes that use a bed or even a dome of salt.

This has three effects that I am aware of-

1- It salts the food obviously. This isn't necessarily a reason all by itself. As you noticed salt is just as easily added later.

2- It keeps the food off the pan. In the case of fish this can make for easier service.

3- The salt becomes part of the cooking medium. It stores and releases the heat of the oven producing slower and more even heating. This is the primary reason recipes call for roasting on salt.

Unless your oven is guilty of gross thermal irregularities, I am skeptical that baking potatoes on salt will make much of a difference at all. It might might make for an interesting presentation at the table.

So here's something I learned in high school that could be of some value. If you have two densities of something separated by something that allows water to flow through, like the skin on your potato, it will try to reach an equilibrium. So in this case you have two different areas with different salt densities. In the potato there is likely little sodium, so water will flow into the potato (with the salt) to try to reach equilibrium. Google semi permeable membranes if you want a more in depth explanation.

In the first Master Class of the 1st season of Master Chef Australia, they made a baked potato--baked on a bed of coarse salt. The reason for this, as explained by one of the Head Chef/Judges, was in so that as the potato cooked (I'm assuming here they baked it unwrapped, since it was was previously done), and released moisture, the moisture would have somewhere to go--into the salt--and you don't end up with a potato with a wet spot on the bottom.

Just my 2 cents: The salt does not create a salty taste at all to the potato. The salt is used to create a type of a dutch oven and cooks a fluffy inside and a crispy skin. Potatoes are cooked like this at Red Lobster. At one time this recipe was called "The Worlds Best Baked Potato" before the internet. If I remember right the temp you cook at is a little higher then 400, maybe 410 degrees or 425 for 1 hour and 15 minutes or something like that.