For me, I would use lactate as confirmation that I'm hitting the right paces and threshold zones. It's basically scientifically checking your work. After, I would just aim for the same "feel" in workouts and if I'm doing it properly, my paces should progress over time.
It's essentially a more accurate representation of the raw physiological response your body is giving you to the effort you're running. The more lactate you're accumulating, the more acidic your blood levels, which lead to more fatigue in the legs. It is literally an indicator of whether you are staying below true threshold.
Absolutely not needed in the broad scale of things, but a good metric to use to create rough ranges for certain rep distances pace-wise, as well as a nice metric to check in with in the future to "validate" fitness improvement. Again, as long as you err on the side of caution and run on the slower side of sub-T, you should be able to be consistent and make progress. After all, consistency is what builds, and you would rather be 5% below the line of sustainability than 5% above.