Games are fun. For most people, board games and video games are an amusing pastime. Even if you're particularly dedicated to being the best at your game of choice, you probably aren't making any money for your efforts.
A few select people can make a living playing games. For example chess grandmasters, eSports champions, and popular twitch streamers. But most of this money is made through sponsorships and tournament prizes.
Players who don't compete on the public stage typically don't make any money. Depending on the game, the best private players probably spend significant money to be competitive.
Additionally, if players ever decide to retire or move on to a different game, all the time and money they put in is just gone.
Enter blockchain. Play-to-earn games are still a fairly new concept. Basically, your in-game currencies and in-game items are moved to blockchain. This allows anyone to earn real assets that they can trade for real money just by playing the game. If you get tired of the game, you can sell your stash and buy into something else.
P.S. this concept can be extended beyond video games. Where else do you see play-to-earn being applied?