Right at the threshold, you behave very differently slightly below, right at and slightly above threshold. For that reason, a workout at 98 % is much easier than a workout that has you at 102 %, much easier than the relatively small difference in watts suggest.
Since you are right at threshold, you are forgoing a lot of adaptations if you do not cross threshold, but stay at or below it.
In addition to what @KorbenDallas writes, there are a number of other benefits:
- You learn to deal with riding hard while lactate is accumulating in your muscles. It is a signal to your body to improve lactate shuttling. So you might blow up, but you might have more time until you do. (That’s what I think @KorbenDallas wanted to get at.)
- It sends signals to your body so that it initiates the right adaptations that lead to a higher FTP. Just think of it: how do you learn/improve very often? You work at the edge of your ability and push a little further until that becomes the new normal. That is, you need to increase the stimulus.