Answer
Tetrapod limbs are believed to have evolved from the pectoral and pelvic fins of lobed fin fishes. Limbs are much stronger than fins since on land body weight has to be lifted against gravity for locomotion, whereas in water body is supported by water and limbs have to just propel the body forward. These limbs are extremely useful for terrestrial organisms because it allows them to use their hindlimbs efficiently for locomotion on land.