Why does the pilot adjust the shape of the wings for takeoff and landing?

During takeoff, the leading and trail edges of the wings (flaps and slats) are lowered usually between 3-15 degrees, Although this causes drag it is still done as it helps the wing create more lift when it is needed most, aswell as aiding the wings to get the aircraft off the ground, It also speeds up the rate of climb and keeps the aircraft a bit more stable while its flying at a slow speed.

If your flying in a 737 (for examples sake)

At 500ft the flaps will be raised a stage (usually from 5 degrees to 2 )

At 1500 they will be taken back another stage ( to 1 degrees)

They are usually kept at one degrees until about 5000ft just to give the aircraft a small lift boost during its climb.

after the aircraft is at around 250 knots (the airspeed limit under 10,000ft) , the flaps will be totally raised so the aircraft is as aerodynamic as it can be and so it can continue its climb at a more gradual and shallow rate.