What is the point of having lots of different flaps settings? This question is about the number of flaps settings on different airliners. One can notice that this varies greatly between models and manufacturers. On a Cessna 172, you usually only have 4 or 5 settings.
But on airliners you have usually more, especially on Boeing.

So I did a quick search and found the following (links point on the image I used):


B777: 7 settings: 0-1-5-15-20-25-30
B737-900: 9 settings: 0-1-2-5-10-15-25-30-40
B787-8: 7 settings: 0-1-5-15-20-25-30
B787-9: 10 (!) settings: 0-1-5-10-15-17-18-20-25-30


On the other hand, Airbus seems to stick to the same scheme for its models:
 the A320/A330/A350/A380/A330 all have 5 settings: 0-1-2-3-full

The questions are:


When you have 9 or 10 different settings, what is the point of having so much ? Is that really useful ? Does the crew really incrementally uses all of them on approach and landing ?
I mean, if they only step, say from 0 to 5 on approach, then  5 to 15 on long final, and then 15 to 30 on short final, then the manufacturer could have saved some metal.
Boeing planes have more settings than Airbus. Is there any particular design reason behind that ?
Airbus doesn't designate the different positions as angles, but as incremental numbers. Anybody knows why they did that design choice?
1a. On take off, with a light airplane you may select less flaps to reduce the additional drag and save some fuel. Max takeoff flaps are reserved only for short runways and high loads. Fuel savings are very important nowadays. 

1b. On approach, less flaps produce less noise and may also allow a higher approach speed (VFE is the maximum flaps extended speed and may be different for different flap settings). Maximum flaps are set only later in the approach when the plane has stabilized at the final approach speed. 

1c. The flap setting has an effect on the airplane's pitch angle. A certain flap setting may help to minimize the tailstrike risk. The longer B787-9 has nine settings, while the shorter 787-8 has six.     


I learned the Boeing numbers are not true angles either. For example, settings 1 and 5 may only affect the leading edge slats. At higher settings they approximate the actual angles (which are anyway somewhat misleading in the case of today's multi-part Fowler flaps, which move both down and backwards). 


https://www.flightglobal.com/news/articles/farnborough-aero-secrets-of-boeings-new-dreamliner-401784/

http://www.boeing.com/commercial/aeromagazine/articles/qtr_4_08/pdfs/AERO_Q408_article05.pdf