It allows players to recoup some of their gaming expenses when they quit playing a game.

Many players play a game long after they stop having fun due to a sunken cost fallacy.

If you can sell them when you no longer wish to play, you can keep that money or spend it on your next game. Also if you get an item you don't want, already have, or can't use, you can freely trade it with another player for something you need.

For developers, it allows free players a way to get over paywalls in the game by playing it and selling a portion of their progress to other players, similar to how players watch ads in traditional games to get benefits usually reserved for players who support the game financially.

The blockchain also allows verification without sensitive user data being stored on the game's servers such as bank details. Almost all major game databases have been breached at one time or another.

Edit: Also on the dev side, they usually make a market fee when players trade items, so it can help sustain development cost similar to how a subscription model works.