The secondary flare is necessary for class II cast gold inlay preparation for several reasons, but basically the direction of the flare results in:
Secondary flare:
It is a flat plane superimposed peripherally to the primary flare. Advantages are:

Secondary flare ensures cavosurface margins to extend into embrasures (self-cleansing area).
Permits easy burnishing and finishing of the restoration.
Produces more obtuse-angled and stronger cavosurface margin.

In class II cast gold inlay, proximal box should have flares so as to have the metal of 30-40° which is burnishable.
Good aesthetics
60° marginal metal which is burnishable
40° marginal metal which is burnishable
10° marginal metal which is burnishable