Many modern integrated circuits and semi-conductor devices can be destroyed or damaged by discharges of less than 200 volts, and the highly insulated nature of these circuits means that static charges can build up much faster than they can be disipated, to a damaging level. Older technologies were “leakier”; they would bleed away static before it became a problem, and could withstand higher voltages. A static discharge that you can feel is probably at least 500 volts, and if you see a spark, 1000. So, less static than you can feel can wreck a valuable electronic device, and you may never know hat happened. Now, these grounding straps connect the technician to the device, to prevent any static buildup between the two.