Building up complicated time series transformations by composing simple functions helps me be sure I'm doing what I think I'm doing. Since the transformations are tacit expressions that don't define specific parameters they are then very easy to re-use and combine. And I have some combinators that can apply the functions in pretty flexible ways. Also, since its all lazy-evaluated on a per-column level I can work with huge tables, but only end up operating on the subset I need.