Why do I have to add a CRM variable to my fields?

A variable is a key that allows you to process datas you collected. 

Let's take an example : in one campaign you ask the participant to give his "Firstname" and in another form "Your first name". Both answers contain the same information and have to appear in the same column of your database. 

Therefore it is important to associate a variable to each field of your form in order to harmonize your entire database. 

You can also use those variable in an exit screen for example : "Your score is {score}" to display the score of the participants.