Ground infrastructure.

E.g. for the 777X it fits in Category E. Without folding tips it would be in Category F. Less gates can accommodate Category F.

https://skybrary.aero/articles/icao-aerodrome-reference-code

Yeah, this is the first place my mind went with it. Aside from gate size category, folding wings could tighten up an overall airport like an aircraft carrier. Return to human-scale non-suburban airports (rip Mueller, Meigs et al).

The second place was the XB-70:

  The XB-70 was designed to use compression lift, a phenomenon also referred to 
  as Mach wave riding, which was demonstrated with the help of wind-tunnel 
  tests. It was shown to provide an increase in total lift at supersonic speeds 
  as wingtips folded downward, and would capture the shock waves generated by 
  the aircraft. Wing lift could therefore significantly increase by up to 30% 
  without any drag penalty.