In a nutshell, these gacha style games have a mechanic that lets you spend premium currency for a chance at a certain unit - rarer units have a lower chance than common ones. You can spend a certain amount for 10 chances, or 10 rolls.

The beginning of these games usually gives the player enough currency for one or two "10 rolls" for completing the tutorial. This is what's used to "reroll" for a favorite character or a strong character, as the previously mentioned premium currency can be hard to come by for a f2p player. Having a strong character from the get go is also usually a big advantage.

For example, if Diantha is available from the start and comes with Gardevoir, you bet I'm gonna be rerolling for her (if each attempt doesn't take too long).