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!
Bear in mind that due to the changes in our policies, we no longer recover lost guest accounts. You can read more about it here: New Policy - NO CHANGES with the accounts (unlinks, email change, lost account recovery).
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 game specific instructions please read: How to upgrade a guest account