As speed is reduced, a higher angle of attack is required to maintain straight and level flight. The pilot increases the angle of attack by pitching the nose up, but this can significantly reduce the pilot’s ability to see ahead.
By extending the flaps, the pilot can increase lift without needing an excessively high nose attitude and forward visibility can be greatly improved.
The increase in lift from the flaps is always accompanied by an increase in drag so more power will be required to maintain the same airspeed.