Before you want to sell your game you have to finish it and you need people to be aware that he exists.
-The Arcade can be used to host your work in progress, you can ask and get feedback from people browsing the Arcade.
-You can do small games and host them on the Arcade to show people browsing the Arcade your style and the quality of your games.
-You can host a demo of your game to attract people to your game.
-The arcade doesn't have ads, people who don't like ads will play your game on the Arcade for this reason.
Or you want to share a good game that you have done without thinking about the $2 you will earn in 5 years thanks to the ads
(you won't be able to really earn this money because there will be a minimum $25 requested to get paid)
Personally I have tried the Arcade but I had a bug with the musics and when I updated the game it didn't update.
I removed my game and made it gamejolt exclusive without ads. (If one of my game attracts thousands of players I will think about the ads of course )
And I like when the player can earn trophies , you can't with the Arcade right now.