I/O devices are much slower than memory. Consider a common input device, the keyboard. Typing at 120 words per minute is equivalent to 10 characters per second, or 100 milliseconds between each character. The Raspberry Pi 3 CPU runs at 1.2 GHz, executing approximately 200 million instructions during that time. And the time intervals between keystrokes are very inconsistent. Many will be much longer than this.
Even a magnetic disk is very slow compared to memory. What if the byte that needs to be read has just passed under the read/write head on a disk that is rotating at 7200 RPM? The system must wait for a full revolution of the disk, which takes 8.33 milliseconds. As with waiting for keystrokes, there is a great deal of variability in the rotational delay between reads from the disk.
In addition to being much slower, I/O devices exhibit much more variance in their timing. Some people type very fast on a keyboard, some very slow. The required byte on a magnetic disk might be just coming up to the read/right head, or it may have just passed. We need a mechanism to determine whether an input device has a byte ready for our program to read, and whether an output device is ready to accept a byte that is sent to it.