Given a principal $G$ bundle $P(M,G)$ and a manifold $F$ with an action of $G$ on it from left, we construct a fiber bundle over $M$ with fiber $F$ and call this the associated fiber bundle for $P(M,G)$.


I do not get the motivation behind the construction given in Kobayashi and Nomizu which I will write down below.


Idea is to construct a fiber bundle with fibre $F$ i.e., we need to construct a smooth manifold $E$ and a smooth map $\pi\_E:E\rightarrow M$ that gives a fiber bundle with fiber $F$.


Kobayashi's proof goes as follows :


They consider the product manifold $P\times F$ with an action of $G$ as $g.(u,\xi)=(ug,g^{-1}\xi)$. Then they consider the quotient space $(P\times F)/G$ and call this $E$.


Consider the projection map $P\times F\rightarrow M$ defined as $(p,\xi)\mapsto \pi(p)$.


This induces a map $\pi\_E:E=(P\times F)/G\rightarrow M$. As $P\rightarrow M$ is a principal $G$ bundle given $x\in M$ there exists an open set $U$ containing $x$ and a local trivialization $\pi^{-1}(U)\rightarrow U\times G$. They then give a bijection $\pi\_E^{-1}(U)\rightarrow U\times F$ and give a smooth structure on $E$ so that these bijections are difeomorphisms. Then, they call $(E,\pi\_E,M,P,F)$ the fiber bundle associated to principal $G$ bundle.


I am trying to understand the motivation for the above construction.


Suppose $F=H$, a Lie group and the action of $G$ on $H$ is given by a morphism of Lie group $\phi:G\rightarrow H$ with $G\times H\rightarrow H$ given by $(g,h)\mapsto \phi(g)^{-1}h$ do get a principal $H$ bundle in above construction?


Edit: I think above content looks like it is asking why do we need the construction of associated fiber bundles. No, what I am asking is, suppose I have a Principal $G$ bundle $P(M,G)$ with an action of $G$ on a manifold $F$ and I want to associate some fiber bundle on $M$ with fiber $F$. Then, **what suggests you to think of above construction**? How does it occur naturally? Are there any other properties of Fiber bundles I should have in mind which suggest this way of construction?



Motivation for construction of associated fiber bundle from a principal bundle
This construction reverses the construction of the frame bundle from a vector bundle (e.g., the tangent bundle). The idea is that each point $f \in F\_p$ in the frame bundle of a vector bundle $E$ is, by definition a basis of $E\_p$. This therefore defines a natural map of $F \times \mathbb{R}^k \rightarrow E$, where $k$ is the rank of $E$. The inverse image of any $e\_p \in E$ is naturally isomorphic to $G$, so the quotient $(F\times \mathbb{R}^k)/G$ is a vector bundle isomorphic to $E$. 


