Salt included during the cooking process is different to salt sprinkled on top of a dish. In the cooking of onions for curry, I believe (rightly or wrongly), that it assists in the "melting" process. Someone with more experience or food science knowledge may be able to add to this, or refute it if necessary.
Many chef's add some salt early in a dish preparation and then taste and adjust seasonings closer to the end. Not just in curries.