Why do some aircraft have pods on their wing tips?
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