Differential operators are badly discontinuous in general, and not defined for all functions. This was recognized as a problem early in the study of PDEs of classical Math/Physics. However, it was found that the inverse problems written as Fredholm integral equations gave rise to operators that are very continuous, and, in modern terms, often compact. This has to do with the Arzela-Ascoli Theorem for equicontinuous families of functions, which goes back to around 1880. Integral operators, such as the Poisson integral, etc., and resolvents for $PDE's$, often map bounded sequences to equicontinuous sequences (which have convergent subsequences.)
A lot of results of such analysis, including existence of solutions, the Fredholm alternative, discrete spectrum, etc., were successfully summarized in F. Riesz's abstract setting of compact operators. The Riesz abstraction was so successful and so clean that the subject is still taught in virtual the same way it was originally presented in 1918.
After the subject of operator algebras began to develop, it was realized that the compact operators are a left and right ideal, and that Fredholm operators could be viewed as invertible modulo the ideal of compact operators. So the ideas coming out of Fredholm's late 19th century work on integral equations continued to bear fruit in more modern settings. Atiyah-Singer index theory then connected Fredholm indices with topological ones. I doubt that this is the end of the story connecting Compact Operators, Physics, differential/integral equations, Functional Analysis, Operator Algebras, Pseudo-Differential operators, Fredholm Indices, Topological Indices, and Geometry.
