while the lay person understanding of lactate and its metabolic role is often poorly grasped (witness all the hype around the demon lactic acid) there is a copious amount of literature written in layman accessible terms on the subject. We summarized much of it in Training for the New Alpinism and in Training for the Uphill Athlete
A very non-rigorous summary here:
Lactate serves a vital metabolic role as a fuel in the slow twitch muscle fibers. It’s transport out of muscle fibers where it is being produced and into muscle fibers where if can be consumed was termed “lactate shuttle” by George Brooks in the early 80s and remains one of the more important developments in the understanding of the workings of endurance.
Of course this conversation can go many, many layers deeper but for most athletes and coaches, having even this basic grasp can help then understand the principles that we and most successful endurance training espouse.