The reason is that maximum take-off weight is far greater than maximum landing weight.
A mid-range, single-aisle aircraft such as the Boeing 737-800 has a maximum fuel load of about 20,000 kilograms at take-off, which accounts for a quarter of its maximum take-off weight of about 80,000 kilograms.
However, the maximum landing weight of that aircraft is about 66,000 kilograms, 14,000 kilograms less than its maximum take-off weight.
If a problem was to develop that required the aircraft to land anywhere short of its destination the pilot needs to dump fuel to the point where the total weight of the aircraft is below the maximum landing weight.
If the aircraft's mass on landing was in excess of its maximum landing weight the undercarriage could collapse.
Fuel is usually dumped at sea or over a rural area, ideally at an altitude of about 2000 metres, which allows fuel to dissipate as vapour instead of raining down to the ground.
Fuel is vented through valves on the trailing edge of the wings, well away from the engines to avoid any possibility of combustion.
See also: Can an aircraft land in zero visibility?