Diablo – Why used forked lightning in diablo 3

According to Bashiok:

...faster speed weapons consume more resource but may do more damage. Channeled skills might still be better with slower weapons though because on a channeled skill, you have as much mobility as you want, so the "mobility" and "overkill" advantages of faster weapons are eroded with channeled abilities, so all you’re left with is increased mana cost for increased damage.

So yes, a faster weapon will cause more skills to tick faster, resulting in you dealing more damage over time, but will result in you using up more resources as you are channeling faster.

Channeling mechanics

Let's use Ray of Frost as an example,

Cost: 20 Arcane Power

Effect: Channel a beam of frost that deals 215% weapon damage as Cold.

Since this is a channeled skill. The resource cost ticks by 10 and drains that much Arcane Power at twice the speed of your weapon. If your attack speed is 1 attack per second, instead of draining 20 Arcane Power once per second, it will drain 10 Arcane Power every half seconds.

Additionally, Since Ray of Frost is a channeled spell. It is broken down into many small attacks that deal a total of 215% weapon damage based on your attack speed. If your attack speed is 1 attack per second, it will deal a total of 215% weapon damage per second.

tl;dr: The faster the weapon attack speed, the faster the drain; slower the weapon attack speed, slower the drain.

Damage-dealing

Assuming two weapons have the same damage, the faster weapon will use up resources faster than the slower one when channeling, but the faster weapon will take less time to do full damage than the slower one.

• An axe at 15 damage and 1.0 attack speed will drain slower. (15 DPS)

• A wand at 15 damage and 1.5 attack speed will drain faster, but will do
  the same damage faster. (22.5 DPS)

I've been playing a monk and, though the in-game skill doesn't appear to have this text, both the official Battle.net builder and the Diablo 3 database contain the following description for Fists of Thunder:

Generate: 6 Spirit per attack

Unleash a series of extremely fast punches that deal 110% weapon damage as Lightning. Every third hit deals damage to all enemies in front of you and knocks them back a short distance. Generates Spirit faster than other Spirit-generating skills due to the high attack speed.

It appears that the lightning animation is used to indicate the small AoE lightning damage and knockback in front of you on the third hit.