We have to main parts in the expression above: A and B. Each of those is doing a calculation. Now, with the markings above, reading the expression is much simpler. The whole expression means this:
=IF(A>B, A, B)
All the above expression is saying is that if A is bigger than B, then return A, otherwise B. Now it is much simpler to read it because we split the repetitive parts into sections. That is what exactly the DAX variable is for.
Readability is not the only benefit, however. Reza has more.