Take a look at this simple illustration of variable scope within a function and how to return a simple value:
Write-Host "Y is $y inside the function"
$y = 10
Write-Host "Y is $y before calling function"
$x = MakeNumberBigger
Write-Host "Y is $y after calling function"
Write-Host "X is $x after being return from function"
The output will look like this:
Y is 10 before calling function
Y is 11 inside the function
Y is 10 after calling function
X is 11 after being returned from function
Showing that the changes made to Y within the function are only in-scope within that function. However, we can capture the value by returning it.