299,792,458 metres per second is the speed of light. Obviously, the metre was invented before the speed of light was discovered, and for a very long time, a "metre" was defined retroactively as the length of the Standard Metre, a bar sitting in France composed of a platinum-iridium alloy. The original definition was something approximating one ten-millionth of the distance from the North Pole to the Equator, and it was created for the purposes of having a single unified measurement that would be used across the board. Now that we know the precise speed of light, it's used as the definition of the metre because the speed of light will never change where physical objects do over time.
The metre is certainly arbitrary, like any unit of measurement, but it's arbitrary over a series of tens and therefore fits with the overwhelmingly standardly-used base-10 number system, avoiding weird-as-**** conversions and units. A centimetre is easy to approximate; take a ruler, and measure your fingers until you find something close enough that you like, then use that every time. The width of one of my fingers is close enough to a centimetre to be getting on with if (for some reason) I ever need to approximate one.