There are nominal voltages and 100, 120, 208, 240, 277 are popular in he US. You haven't heard about 208 or 277 because these are voltages available in a 3 phase industrial environment. 100 isn't a US voltage at all, but it happens to be a standard in some areas of Japan. 50 hz and 60 hz power systems are common and therefore some equipment is marked 50/60 Hz. Universal power supplies are available where the voltage can be anywhere from 95 to 285 VAC.

Part of it's history or even folklore. I lot of devices were specified to be tested at 117V.
You'd probably like 120 V +-10% for a piece of eqiupment and 120 V +-5% for the actual power. The power company cannot regulate the power into everyone's home separately, there are losses that must be taken into account.

Also voltages vary around the world. I don't have a reference handy.

Here is some old antique equipment using the 110/220 power source.

There is generally enough sloppiness in the design to let everything work out. Something designed to operate on 100 V won't operate on 120V.