Let's say you want to connect a switch to an FPGA.
You might build something like that:
But the problem with mechanical switches is that... they bounce! When you press the push-button, you might get lucky and get something like that:
That's rare. More common are these...
Now let's say you implement a counter inside your FPGA.
You add a display to see what's going on.
Power-on, things look good.
You press the push-button 10 times, and...