On a clean disk, you can seek forever.
—Thomas B. Steel, Jr.
Computers are good at computing. This tautology is more profound than it appears. If we only had to sit and chew up the CPU cycles and reference RAM as needed, life would be easy.
A computer that only sits and thinks to itself is of little use, however. Sooner or later we have to get information into it and out of it, and that is where life becomes more difficult.
Several things make I/O complicated. First, input and output are rather different things, but we naturally lump them together. Second, the varieties of I/O operations (and their usages) are as diverse as species of insects.
History has seen such devices as drums, paper tapes, magnetic tapes, ...