What does the complexity class $\mathsf{XP}$ stand for?
There is a slightly different description for XP which I personally find less misleading: "Polynomial time for each parameter". I believe the zoo page uses FPT instead of P for some formal reasons (parameterized problems are sometimes defined as subsets of $\Sigma^*\times\Pi^*$, classical problems as subsets of $\Sigma^*$). It can be noted that both formulations describe non-uniform XP, while the time bound $O(n^{f(k)})$ refers to uniform XP. In any case, the important difference is that for XP, the exponent of $n$ may depend on the parameter, for FPT it mustn't. 

The practical importance of XP is as a door to parameterized complexity analysis. Once a problem is recognized as belonging to XP it makes sense to investigate its exact parameterized complexity. If it does not even belong to XP, parameterized analysis doesn't make much sense. It can also be argued that in that case the parameterization is not meaningful.