Boxing is often good for self-defense because it teaches a high level of fitness, quick movement, and a high level of reflexes. These are all critically important when in a self-defense situation. As most fights usually remain on the feet with swinging arms, knowing how to out-box an opponent is often very effective. But if there's even a slight chance of getting tripped up and ending up on the ground, your Boxing skills can become completely useless. That's why having some ground game is usually very necessary.