What is the function of salt in chocolate?

Salt tends to enhance tastes, and make flavors pop, and is used in many foods for this purpose. Even most sweet foods, desserts or baked goods, will have some amounts of salt added for this purpose. Some certain kinds of salts - including many kinds of sea salt - are also called out for additional tastes or textures, caused by trace minerals or crystallization patterns - such are usually known as finishing salts, since dissolving looses many properties (including texture, and some elements of taste which are easier distinguished in concentration). I would assume Atlantic salt is one of these finishing sea salts.

Also, contrasting flavors and/or textures are fairly common in cooking - in this case, sweet and salty. This often seen in savory dishes (sauces for meats, for example) where sweetness is added - sometimes quite a bit - even though the resulting dishes are generally considered savory. And while such foods quite different from foods without that added sweetness, if well balanced they can be well received.

A sweet-with-salt, like salted chocolates, or salted caramels, is very different from a pure sweet taste. It isn't inherently better or worse, just different. Too much sweet can overwhelm the tongue (one reason milk, rather than juice, is favored with many kinds of desserts), and adding salt to that sweet cuts the sweetness, adds flavor and contrast, and also adds texture as large grains of finishing salt, often sea salt, are used rather than fine dissolved salt. The result can be very pleasant to some.

Also, while your example is a sweet with salt added for contrast, it may be worth noting that chocolate is not itself inherently sweet, but generally has a more bitter flavor profile (unsweetened chocolate) and so can be easily used in savory dishes - for example, Mexican mole sauces. Sweet chocolate is very common, yes, and tastes very good, but the base flavor can certainly take salt without trouble - and the less usual pairing can increase the novelty factor and the pleasure one takes in the salt and chocolate pairing.

Salt is a bit special because of its action on the tongue. The sea salt, in this case, is a major ingredient added to be intended to be an independent flavor, but it is also a flavor enhancer. Often in chocolate, large crystal sea salt in used to give a salt crunch and taste.

So, for this purpose, salt can be considered a complementary flavor enhancing the flavor of both the caramel and chocolate. Instead of caramel, chile peppers or orange peel could be used with the sea salt. That is while you'd be hard-pressed to find Anchovies and Sea-Salt Dark Chocolate.

You'll also find concept of flavor opposites in South East Asian cuisine like Salty(Salt)/Sweet(Sugar) and Spicy(Chile)/Cooling(Mint).

While this is a different question, the answer is essentially the same as https://cooking.stackexchange.com/a/17264/6279.

Salt is both a seasoning, with it's own unique flavor and a catalyst for other flavors. The collection of taste buds on your tongue are not 'all created equal' while some react to acidic, sweet, salty flavors others do not. When salt comes in contact with the tongue not only can you taste it with the 'salt sensitive' taste buds but it also triggers a reaction with the 'sweet' sensitive taste buds to make them 'more receptive' to sweet flavors...

For a more thorough description, with visual aids and all, Check out "The Ballad of Salty & Sweet" from Alton Brown.