After I beat a level on a difficulty, that difficulty gets a check mark atop it. I have confirmed that if I beat a level on two difficulties, BOTH get check marks (as opposed to just the hardest).
Is there any benefit to beating levels on all difficulties (other than check marks for the sake of check marks)?
This has been answered on gamefaqs, here you go :
No, there isn’t a reward. However, once you beat Legend Mode you’ll unlock Hero Difficulty, which gives you a Medal if you beat everything in Hero Difficulty.
There isn’t anything gained by having the Medal.