Q:

Which purpose do the checkmark Candies serve?

There are candies with a checkmark on them, which purpose do they have? 

A:

Seems that this candy is a so-called lucky candy booster.
Its available after you complete level 130.
Seems that it's only available on "collect Orders" levels (according to some sources).
When you match it with other candies of the same colour it will turn into something useful (typically something that helps you clearing the level).
The Candy that looks like an egg with a question mark on it is Mystery Candy. If you match them, they can give you either something good or bad.
Source:
Candy Crush Wikia - Lucky Candy

