It was basically the original functionality/selling point of the coin.

Dash launched as Xcoin in January of 2014. Very soon (days, weeks?) it was rebranded to Darkcoin, whose first feature was privacy. It was only later that Darkcoin rebranded to Dash and focused on being a general-purpose digital cash for the masses.

But privacy is the first major feature, before even instant transactions or masternodes.