For a finitely generated group $G$ with generating set $S$, one can define the word metric. Using this, we are able to define the sequence $\sigma(n)$ by 
 $ \sigma(n) = |\mathcal{S}(e,n)| $, which is the number of elements that are contained in the sphere at a distance $n$ of the identity element (ie. All elements in $G$ that are at a distance precisely $n$ of the identity). Consequently one can define the growth series $\mathcal{S}(n) = \sum\_{n\ge 0} \sigma(n) z^n$. One can then ask if this formal power series is rational (ie. it is the power series associated to a rational function). 


I have two questions concerning this:


1. Why is it interesting to ask this question? (ie. Why is the fact that the formal power series is rational interesting?)
2. If one would take two different generating sets $S$ and $S'$, is it possible that the the growth series coming from $S$ is rational and the one coming from $S'$ is not?


Thank you for your help. 



When are growth series rational?
An answer to your second question: Stoll constructed many 2-step nilpotent groups such that there exist generating sets $S$ and $S'$ such that the growth series with respect to $S$ is rational and with respect to $S'$ is transcendental. See


M. Stoll, Rational and transcendental growth series for the higher Heisenberg groups, Invent. Math. 126 (1) (1996), 85–109.


As for your first question, I don't know if it is intrinsically interesting, and I am not aware of any applications of these results. However, what is interesting are the vast array of tools and techniques that are used to study it. It is a good proving ground for many ideas in geometric group theory (especially those related to things like regular languages). There is an enormous literature on this topic. The introduction to my paper 


A. Putman,
The rationality of sol manifolds
J. Algebra 304 (1) (2006) 190-215.


summarizes most of the papers concerning it that I am aware of. It can be downloaded from my webpage [here](http://www.nd.edu/~andyp/papers/). The only ones I know about that came out after it are


1. My student Corey Bregman's paper "Rational Growth and Almost Convexity of Higher-Dimensional Torus Bundles", available [here](http://arxiv.org/abs/1503.06820).
2. Duchin-Shapiro's paper "Rational growth in the Heisenberg group", available [here](http://arxiv.org/abs/1411.4201).


