Not regularly but I used it recently at work to compute symbolically the sum of a non-trivial series. Turned out the sum has a closed form, which allowed me to replace an O(n) for loop with an O(1) bunch of additions, multiplications and divisions. Felt nice.
Sure, here's one: given a list of n items, compute the sum of pairwise distances between every pair of items.
The first item has distance 1 from the second, 2 from the third, ..., n-1 from the last. Similarly, the second item has distance 1 from the third, 2 from the fourth, ..., n-1 from the last. And so on until the (n-1)-th item.
