I heard yet another theory on why turbo button was introduced: some early networking stuff failed to work at higher speeds.
BTW, in certain book I saw recommendation to turn off turbo when running commands like FORMAT, DISKCOPY, DISKCOMP...
And I had at least one machine where it was handled automagically (in BIOS?): even though the button was set to turbo, during diskette operations the system switched itself to de-turbo, it was clearly visible via the turbo LED.
More likely you're running games that are clocked and not dependant on system timing to function properly. Else how does the system know when to slow down, and when not?
I've got games from 1985 that work fine on it. Go any earlier than that and nope.
"A push of the button on the front panel (next to the stylized racing stripes) lights the Turbo logo in red and shifts the computer into Turbo drive" (InfoWorld, May 14th, 1984):The Eagle Turbo XL of 1984 had a button on the front panel that changed speed. No fancy name for the button yet.