Using flaps during landing can help in a variety of ways: 
Producing greater lift and allowing for a lower landing speed
Producing greater drag, allowing a steeper descent angle without an increase in airspeed
Reducing the length of the landing roll
Generally speaking, flap settings up to 15 degrees increase lift with minimal drag; settings above 15 degrees produce a large increase in drag.
Different manufacturers recommend different flap settings for take-offs and landings with varying conditions- consult the AFM for your aircraft for the official word on the use of flaps.