This question was posed recently: "Why would a martial artist need an AR15?"
- You're right. A marital Artist doesn't "need" an AR15.
- A martial artist can magically defeat any attacker - regardless of size or gender.
- A martial artist can defeat multiple attackers.
- A martial artist can defend a friend or family member from a distance.
- A martial artist can single-handedly thwart a tyrannical government.
That black belt is a magic talisman. It causes things to happen that cannot happen otherwise. It imbues a person with mystical powers.
More realistically - this is all satire and the above is laced with sarcasm. You decide.