Artwork, jewelry, and other collectibles can sometimes be viewed as an alternative way to store value outside of the normal financial system. After a certain level of wealth, you want to diversify how your money is actually stored to guard against certain kinds of risks. Storing money inside alternative objects that hold value decently well is one way to do that. I believe Joshua Kennon has referred to that in some articles.
Collectibles are probably not a good choice for a standard early retirement plan, unless you have exceptional knowledge of the field. For instance, I have an acquaintance who makes lots of money trading yu-gi-oh cards because he is an expert on the subject matter and that allows him to exploit inefficiencies in the market.
You might be surprised how analytic the yu-gi-oh trading field is. The valuation of a given card depends on the tournament scene, including how strong the card is in play, but also how likely the card is to be banned, how easy the card is to obtain, and a variety of other factors. By correctly calculating the intrinsic value of a card, my acquaintance is sometimes able to buy a huge volume of them at a deep discount and then control a significant ratio of the supply, so that once the true value is recognised by the market, supply is artificially low and he is in a position to charge a premium.
If you wanted to get into collectibles, I would suggest picking a fairly obscure field where you could be an expert.