Mii Channel
On Wii Sports you play with them in order to get their skill level higher. And they look better when you are playing with them. On Wii Play you play with them in the different games to earn higher medals in the games and to play against a friend. And in Mario Kart Wii you can play with your Mii (once you unlock it) to race with.
On the Check Mii Out Channel it is different. Instead of playing with them you upload them on there, they will get a star rating which can also affect yours. You can also enter them into contests, if you enter your Mii will get a rating between 1st Level and 10th Level. If you vote on Miis you will get a rating on how good your "eye" is meaning how good you are at picking ranking Miis. If your Mii makes it to the Top 50 it will be featured on the channel.
There are lots of different ways you can edit your Mii. You can choose: hair style, hair color, eyebrow style, eyebrow color, moustache (or not), moustache color, different types of beards, beard color, mole (or not), eye type, eye color, different types of wrinkles, skin color, lip shape, lip color, sunglasses, glasses, and tons of other cool things. You can also make your Mii taller or shorter and skinnier or fatter. You also get to name your Mii and set a birthday for it.
Basically, Miis play a very important role in the Wii. And without them the Wii wouldn't be as fun, nor would it have as much features.
