Lactate is usable fuel. The waste-product story leaves out most of the story.
Your body makes lactate during exercise, and other tissues can take it up and use it. It can also be produced when oxygen is available. So seeing lactate rise does not mean your muscles have suddenly switched to a completely oxygen-free system.
A blood reading reflects the balance between lactate entering and leaving the blood. It is not a direct readout of how much damage you have done, how brave you were, or how good the session must have been.
That matters when runners start chasing a particular number without understanding the test, the timing or the training purpose. A useful measurement needs a useful question behind it.
You do not need a lactate meter to train well. But if you use one, learn what the number can tell you before letting it dictate the session.
Read the caption again before your next discussion about “flushing out waste”.