Licenses are required, else users don't know what they can do with your asset!
https://itch.io/t/402794/warning-for-authors-of-unlicensed-assets
Viewing post in DUNGEON TILES + PLAYER AND 2 ENEMIES comments
You just provide the license, the users who download your files use the license. Basically a license tells the people what they can do with the assets you provide. For example: Can they use your files in commercial products/games? Can they be modified? Can they be redistributed on other sites? And so on...
There are many predefined licenses available, e.g. the Creative Commons. You can check pretty much any other download on itch.io to see what licenses they use.