Suppose that we have a power series: $f(x) = \sum_{k=0}^\infty{c_i x^i}$
Now suppose that we can get a closed form, e.g. a function composed of simple arithmetic of elementary functions, for the first $N+1$ terms of the series: $g(x) = \sum_{k=0}^N{c_i x_i}$
What are the known applications of this? More specifically, I'm looking for the most important/noteworthy application(s). I hope that this avoids being downvoted, as I'm really just trying to get a good intuition of "truncated" series. Any help on asking a better question would be appreciated, because I really would like to get to know this topic better.
Some other questions that asked for an "implementation" or specific series are Constructing finite versions of arbitrary series and Closed form of a partial sum of the power series of exp(x). I'm not looking for how to do it, just what the results could possibly do.