Why do some aircraft have pods on their wing tips? By JetPix [GFDL 1.2 or GFDL 1.2], via Wikimedia Commons

Why do some aircraft have pods on their wing tips, like this Cessna 414?
Wingtip pods or winglets differ in installation reason from aircraft to aircraft. Here is a non exhaustive list of possible reasons:


Fuel tanks
Whitcomb bodies to enhance the area rule of the aircraft also known as Kuchemann Carrots
Vortex generators and relievers - used to enhance aircraft aerodynamics and modify fuel consumption
Mass balance - in particular on forward swept wing aircraft
Housings for sensors or landing gear