If you are looking to try a game out for the first time, a guest account might be great for you, since you get to play without getting attached. But if you want to progress in the game and make sure you won't lose it, it's highly advisable to upgrade your guest account, in order to save it. Besides, you can get some amazing rewards in the process!
Playing with a guest account leads to lost progress very easily, because these accounts are saved in your device cache and, if something happens to it, the account is lost. It is impossible to recover guest accounts, most times, so this is always something you want to avoid.
There are several ways which you can save your account Different games have different methods, but, generally, going to the settings and pressing the "login" to a Facebook account will work.
For ways to upgrade your guest account, be sure to check the section of the specific game you play.