Because in Python, and programming generally, we need a place to keep our results for a while until we need them. Keep in mind that whatever you compute with a program, you lose it if you don't store it somewhere.
So variables are just that. A way to store information so that you can use it later on in your program and thus they play a central role in development. Except for the case where you are doing something really simple, you will want to use variables to keep things.
As you learn more about programming, you will understand this concept for yourself. Until then, here are some examples.
Example 1 : Numerical Sequence
Let's say that we write a program to compute a sequence of numbers up to a number N. The most famous is the fibonacci sequence for which we know that
\[x_{n-1}+x_{n}=x_{n+1}, x_{0}=1=x_{1}\]
To do that with a program, we need to store somewhere the two last values \[x_{n+1}, x_{n}\] so that we can use them to compute the next number. There is no way that we can continue calculating if we can't store the results somewhere.
Example 2 : Generality of input
Let's say that we want to write a program in Python to battle against internet trolls who write everything in CAPS. So, our program should take as input a phrase in caps and turn into something in lowercase. Since we are talking about a general phrase here, we have to store it somewhere so that then we can give it to the relevant function in Python.
Example 3 : Ease of programming.
Let's say that you have to solve the following problem in Python.
a) Import the math library and compute the values of sine, cosine and tan at 18.849555921538759430775860299677017305183016396250634
Now there are of course many ways of solving this problem. Here's one :
import math
math.sin(18.849555921538759430775860299677017305183016396250634)
math.cos(18.849555921538759430775860299677017305183016396250634)
math.tan(18.849555921538759430775860299677017305183016396250634)
and here's another one :
import math
x = 18.849555921538759430775860299677017305183016396250634
math.sin(x)
math.cos(x)
math.tan(x)
The second way is easier to type, easier to read and has one more important advantage. It's general! Just change x and you have the values of functions wherever you like without having to change anything below that line.
I hope those three examples illustrate, even slightly, how important variables are for programming. :)