Jakob touched on this during the video. If the reading is very low but he doesn't feel great, it suggests he is fatigued. So he might scale back his training a touch for the next few days. If the reading is higher than expected, Jakob might have to reduce the intensity of the rest of the workout. If the reading is high at the end of the session, it might mean he has gone harder than planned and might need to scale back training for the rest of the week.
Also, by comparing the readings to data from previous seasons, he can see where he is in within the periodisation process. Jakob and a few other people have mentioned that a greater focus on anaerobic work towards the end of the season, while helping maximise 5k/1500 time, also leads to a degree of aerobic detraining. So if lactate values are higher than expected in a long threshold, but lower in a quicker session like these 400/200 repeats, then Jakob might be further along in his periodisation than desired, so he might focus more on threshold training.