If multiple people have this AddOn in the raid, you can all see each other's threat on the overall of the fight (or on a set master target). People who don't have KTM will not appear on the meter. This is considered useful on many guilds to make sure you are under the Main Tank's aggro.
KLH Threat Meter is an aggregate threat monitor, so will not report threat for multi-mob encounters, unlike the Omen AddOn. Also, Omen can gather threat information from characters using KTM.