As well as instilling a sense of confidence.
The fear of verbal or physical assault can be
moderated.
Competence in physically defending oneself, the ability to protect one's own space and person, can be very
anxiety-reducing.
It can produce a much more "comfortable in your own skin" awareness and confidence.
We can be attacked by varying mental, verbal, and physical strategies.
These attacks are often relentless.
Perhaps not for your average, emotionally driven/"adept" person, but for some of us, it can be exhausting. Heavy.
It is easy to imagine(and has not been contra-indicated by experience) conflict becoming confrontation, especially in the testosterone-fueled world of men.
Things can become physical at the drop of a hat, given the (in)correct circumstances.
Being competent in physical defense removes the top tier fear--- that of taking a beating.
When you feel secure physically, feeling secure mentally will follow.
This can make for healthier, assertive behavior. Calmness.
Training in a martial art, in and of itself, is a form of meditation, and often "pure" meditation practices are utilized and expected as part of the training.
In my life, I have also detested confrontation of any kind, especially physical violence. But, as I have
had to go on to explain in some cases,
that does not mean that I am not exceedingly freaking good at it.
I can't really emphasize how this eases even the most inane of exchanges.
How it instills a calm, secure feeling, from which it is much more comfortable to speak one's mind, one's direction.
I can personally recommend martial arts training for anyone with assertiveness difficulties.
Just familiarizing yourself with tumbling on a mat, or push-hands exercises, can instill confidence--- it breaches, explains the unknown a little.
Of course, the more proficient one is, the more confidence one can have.
I intend to answer the OP, just wanted to say this here, @Nervous Rex .
I'm sure I've posted this before, but I admire it's elegant firmness:
(Said to have hung as a tapestry,
in the Old Shaolin Temple)
"I would rather maim, than kill;
I would rather hurt, than maim;
I would rather intimidate, than hurt;
I would rather avoid, than intimidate."
I studied Karate for almost a decade and I'm still as assertive as a wet noodle!