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
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.