Strength (Resistance Training)
Put Some Power In Your Gloves- Weight Exercises For Boxing
The generating of maximal power through any punch will certainly tell one's opponent they mean business, and this will have a profound psychological effect.
Indeed, developing power will also help to enhance speed and anaerobic fitness.
Speed will improve as muscles become used to pushing out heavier weights (the cornerstone of any power routine), which translates to a faster punch when the comparatively light, 10-14 ounce gloves are laced on.