Linked from
The 40 pages that link to Banach space, each with the reason it gives.
Cauchy criterionRelated: Completeness ensures Cauchy sequences of vectors converge within the space.
Fréchet derivativeRelated: Many central results about Fréchet derivatives assume complete spaces.
Operator theoryRelated: Completeness lets analysts study limits of operator sequences and series.
Fredholm's theoremRelated: The standard operator-theoretic theorem assumes complete spaces.