There are almost limitless reasons for having a decent weak hand jab. A couple of key ones from my (limited) experience.
If my opponent only throws their dominant hand I can disregard the weaker - giving me the following benefits.
- I can move my guard to only cover their dominant hand
- I can focus my actions and timings to counter just one hand - (this is so much easier)
- I comfortably know which way to move when they lunge forward to avoid being hit.
Looking at these in reverse.
- Using both hands (with a variety of punches) means our opponent has a larger area to guard, even with a jab, my left hand attacks from a different angle to my right meaning a solid defence from my right jab won't always stop my left. Whilst stepping around your opponent the gap may open for the weaker hand first, landing a strike with this often allows for a powerful punch with the other hand.
- If they are concentrating on blocking your dominant hand attacks - your weaker hand may be able to land on a regular basis - keeping the scoreboard rolling in your favour if nothing else. Concentrating on attacks from more appendages is more mentally draining - hopefully draining enough that your opponent gets sloppy in later rounds.
- When an opponent ducks and weaves their way out of an attack, naturally they will favour the opponents (generalisation here) left side (as most of their sparring partners will favour hitting with their right) if you are happy to land a powerful shot with your left as they go it will mess with their game plan. They may still favour this side over your dominant hand but they will have to absorb a hit each time.
The main purpose of jabbing with the weaker hand at the higher levels (and especially weights) is so that the stronger hand is poised for the powerful strikes that may end the bout early.