Interesting. I see no one knows what was the purpose of the x command. Unfortunately, QBASIC guide is not clear, and many people believed it was necessary to use that weird syntax. But actually, on older versions of Microsoft Basic, there is a clue: on MSX Basic (that is pretty similar to GWBasic), the x command was supposed to be used like:
DRAW "xa$;"
https://www.msx.org/wiki/DRAWIt made no sense at all neither in GWBasic, nor in QBASIC... but after more than 30 years it's still here, because too many people don't know there is no need for it