How are recursive function calls of Merge stored in memory for this Python GeeksforGeeks Mergesort implementation?

def mergeSort(arr, l, r):
    if l < r:
 
        # Same as (l+r)//2, but avoids overflow for
        # large l and h
        m = l+(r-l)//2
 
        # Sort first and second halves
        mergeSort(arr, l, m)
        mergeSort(arr, m+1, r)
        merge(arr, l, m, r)

I'm a bit confused. I thought that just calling a function wouldn't change the value of arr unless I equated arr with the output of Mergesort ie arr=mergeSort(arr, l, m). What's the difference between calling a function where arr is modified vs calling a function where arr is modified and then subsequently setting arr to the output of the function?