There are no "theories" on this.
It removes current & prevents sweat to come in your hand palms.
Today,
powdered magnesium
is used by weightlifters & such. (Not many ancient, melee weapon fights going on nowadays.)
There may be a secondary ritual; "The battle starts now." Anyhow, that is "secondary." The dust & dirt/gravel serves a very practical purpose.

To bewilder viewers so they question it on imdb.
Say, do you hear that? It's the sound of the Reaper..

Same reason baseball players do it.

its for grip on the sword, he doesn't want it to slip out from sweaty palms, just like rock climbers using chalk for grip. same thing

Shadows and dust. As a soldier and a gladiator, he was that close to death every time he picked up his sword. Also, better grip.
You'll Never Walk Alone

http://www.newsday.com/sports/baseball/mets/rookie-darrell-ceciliani-delivers-for-mets-1.10551010

He uses it like lifters use chalk, for grip.
It might also have become a ritual pre-fighting.

Both
"a malcontent who knows how to spell"

Better grip.

Some people just don't get symbolism.

This is because he is a farmer. The sand and dirt define him. They make him who he is. The land gives him trust and strength.