15/8kV (air/contact discharge) is a typical upper limit, not that higher can't be done, but that commercial purposes rarely require more. See IEC 61000-4-2 for example.
ESD is not simply a voltage, but a type rating which includes a typical waveform, and the network that delivers it, thus both voltage and current are well described. The voltage is the initial charge voltage. The pulse is quite short, <1ns rise, ~50ns width, and the energy fairly modest (10s mJ), enough to get zapped by, but when microscopic elements like semiconductors are subjected to it, it's easily enough current (10A+) to vaporize tiny metal contacts, voltage to break down insulators (nm's of SiO2, etc.), and energy to melt semiconductor junctions into short circuits.
1kV or so, is a fairly typical minimum rating. It may be due to JEDEC standards concerning ICs, but I have no idea as they aren't publicly available. 1kV is enough that exceptional precautions are not needed, but basic precautions such as dissipative containers, ESD mats, wrist straps, etc. are desirable.
ESD is due to the sudden discharge of built-up static. Usually from triboelectric sources like walking over floors, shifting around in office chairs, etc. With low humidity, voltages of several kV are common, with 10kV being possible, usually with "unlucky" materials, or extra effort
ESD only depends upon the voltage difference between objects before discharge. Equalize that, before contact, and you eliminate ESD.
So, you can touch the chassis of equipment you're working on, or the dissipative bags that parts come in, to discharge to something save (the enclosure/container), before touching sensitive parts.
The only reason for grounding, is it's a universal reference. Put equipment on ground, and parts, work surface, and personnel, and by the transitive property, voltage differences go away.
Tim