It's not about variables, but about: how do you store Data
Data is like, your character's x position, for example. If you want characters to move, you add 1 to their X on every tick, then it moves!
But if you want to change your code, you have to go through every single place you wrote the number 1, and change it to 2, if you want to move faster. But what if you change your mind again ? You can create a variable that contains the character's movement speed, so you don'T have to change it every time.
What if you want your character to only move if you press a button ? You can add a condition, "on button is down" to your code and then your character moves when you press the button !!
What if you want your character to move the other way when you press a different button ?
What do you do then? Do you copy and paste your code, but add a - symbol in front of the movement speed variable? What about when you also can move on the Y axis, do you have to write even more code? What about when other objects can move, or can move at different speeds, ??
Do you instead have the code to move your character be on every tick, and instead change the variable, if no button is pressed, the variable for speed is 0, if the button to go left is pressed, the variable changes to a positive value, and to go right, the variable is changed to a negative value. If you make it this way you don'T have to rewrite your code anymore !!!
But there's a problem, pressing the moving key moves everything !!! What can we do about that?
There's this thing called instance variables, every object in construct 3 has "instance variables"
What if, when you press the button, instead of changing the event sheet global variable, you change a single object's instance variable, then you can use the one movement code only once and move everything independently !!
What if you want to add acceleration to your code, how would you go about it?
What would you need to do to be able to do collision detection ?
Using variables is about thinking about these things, and the more you practice the better you get at it, and you can make more sophisticated systems over time !
Get out there and make some cool stuff !!