Here's a look inside a typical modern keyboard. Beneath each key lies a free-floating silicone dome switch that can easily wear out over time or degrade in harsh environments. When depressed, the dome buckles and pushes together two contact-laden plastic membranes, completing the circuit and registering a key press.
For many users, a keyboard like this is probably good enough. After all, if it wears out, it's cheap to replace. But for serious typists, or those who simply appreciate a solid keyboard beneath their fingers, an IBM Model M is, without question, the only way to go.