We could also use "OHM" instead of Ω and "EUR" instead of €, but symbols provide more concise representations of meaning, especially in the case of a combined ON/OFF button. Much easier to fit the universal circle with 1 in it than "ON/OFF". The broken circle with 1 in it is way smaller than "ON/STANDBY". The other two symbols are then necessary to keep the proportions consistent across all four related glyphs.
That's the only argument I've seen for it that makes any sense. I'll counter by saying only "ON" is needed, not "ON/OFF", as the off part is implicit. Same goes for STANDBY.
This has been standard practice for a long time, I'm not just making things up on the fly. BTW, SBY is a standard abbreviation for STANDBY used by the military, if space is a problem. And SBY is a lot more google-able than an icon.
