native ResetTrigger

what does it do?

does it clear the trigger of any events/actions?

No it reset the number of evaluate/execute trigger.
GetTriggerEvalCount and GetTriggerExecCount.

Is it just lame or is there a fitting opportunity to use it?

Avoid to use a variable ? :p

which is actually encouraging the creation of single triggers for each player, e.g. you want to reward a player when he killed 100 units. In order to use TriggerExec/Eval count you would have to make a single trigger for each player.
But yeah, in all single-trigger cases it might actually prove useful :P