One situation I have found it helpful with is when one front tire has traction, and the other has little or none, which is usually only when parallel parking (or unparking) after a snowstorm.
Leave it on, it does no harm in normal driving, and can help in slippery conditions, as it only intervenes when you get wheel spin. However a little wheel spin is a Good Thing when going for maximum acceleration:
So I switch it off as I roll up to the stop sign at the entrance to the parkway.
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