“Extra advertising space for your company logo” and yes that was given as an answer and the guy got it (but that was back in the early days of DJ).
Seriously the interview panel at DJ will want to know what the performance benefits are with winglets as they are pilots and not bean counters. The above link has the answers. DJ is fitting winglets to all their aircraft, some of the older leased aircraft don't have them and never will, has they will be returned to their owners and replaced with newer 737NG's fitted with winglets.
All the best with the interview.