The question may seems very obvious but I don't know the answer right now. I know that De Rham cohomology reveal some properties of the topology of smooth manifolds by finding closed differential $k$-forms $\mathsf{d}\omega=0$ that are not exact $\omega\neq\mathsf{d}\eta$. I wonder
Question: Why non closed differential forms do not play important role for the topology of a manifold?
One can say that because the closed forms work pretty well i.e. the PDE approach to closed and exact forms that asks for solvability of a system of differential equations, then Poincare Lemma enters. All goal of the solvability is based on closedness and exactness of system of differential equations. The other reason is that the set of all non-closed forms, is not a vector space. But these are not a good and convincing reason for non-closed forms not affecting on the topology of the manifold.