What is the reason for starting before the traffic light turns green? I've noticed that a lot of commuters come to a complete stop at traffic lights but go before the light turns green. They start riding as soon as the light in the intersecting direction turns red. I usually get annoyed and assume that this could only reinforce negative opinions held by motorists, but I've suspected that there must be some valid reason for the early start.

This is a separate issue from taking an Idaho stop at a quiet intersection or blatantly running a red light. For rolling stops and blatant running, there is a significant saving in effort from not completely stopping, and there is an understandable, if not necessarily justifiable, tradeoff between efficiency and legality.

There doesn't seem to be the same tradeoff for early starts at traffic lights. In this case, the saved time is only two seconds, and there is no saved effort (since they have already come to a complete stop).

I've seen enough people do this that I'm sure there must be some logic behind the early start. Are commuters really desperate to save two seconds, or is there (as I suspect) some deeper reason for taking an early start at traffic lights? Does this behavior just hurt relations with motorists, or are there safety benefits that make this something that should be pushed for legalization (as with Idaho stops)?
The "early start" allows cyclists to clear the junction ahead of motorists, this is useful for cyclists making off-side turns (right in the UK, left in the US). It also helps cyclists going straight on avoid conflict with motorists making near-side turns (left in the UK, right in the US).

The UK and many places in Europe have Advanced Stop Lines (Highway Code rule 178) at traffic lights:



These allow cyclists to get ahead of motorists without jumping the red light.

For junctions without an Advanced Stop Line you'd have to weigh any benefits of clearing the junction against the potential legal consequences, the likelihood of increased driver hostility and the risk of colliding with a vehicle jumping the lights from another direction.

Personally, I don't jump red lights. On the roads I cycle on I think it would be more dangerous than waiting for them to change.