You can touch a mechanical switch. It acknowledges the switchover with a rich "clack". The operator is informed haptically and acoustically that the switching process was successful. This feedback is missing with a touchscreen: The actuation often takes place before a solid surface is touched and proceeds silently. The operator is left with the uncertainty: Have I pressed yet, and has the system perceived my touch?
Resistive touchscreen technology, which is rarely used today, has one major advantage: You can place a finger on it without triggering a touch event. This only happens when you press further. Current technologies like PCAP and infrared touchscreens completely lack this. Even before touching the surface or entering the light window, a touch event is reported to the system. If the system does not update the screen content accordingly, the user does not receive any spontaneous feedback as to whether his touch was accepted by the system. Audio-visual feedback via the eye (change of screen content) or acoustically via the ear ("click" or "beep") is only perceived in the brain via a detour. Only haptics provides immediate feedback: Where the finger has touched the touchscreen, it vibrates and reports the action directly back to the finger. With the appropriate effort, it is even possible to imitate a mechanical spring-loaded button haptically and acoustically. Haptics enhance ergonomics and make blind operation possible without looking.
Read detailed information about haptics in touchscreens - passive and active haptics, phases of a touch process, measurement of force, selective feedback, advantages and disadvantages of different actuators and system integration.
Note: This white paper is in german.