I should leave this to David but will jump in. Simple doctors, like crystal sets, can not make unmodulated signals audible. Crystal sets were frequently used as backup and emergency receivers for distress purposes so, to insure distress traffic would be readable 500 khz was modulated.

In a way it's a hold over from pre-CW signaling since _spark_ signals (damped waves) were effectively modulated. 

Let's see if I put my foot in it.