Daniels stated purpose for this kind of workout is to improve running economy by improving form. His idea is that when you run at a fast pace, your body tends to naturally streamline some of your running form, getting you up on your toes,
Getting you leaning forward,
Getting your driving your arms forward, getting good knee lift, etc.
So from hat perspective, doing these on shorter rest can actually be counterproductive as you start to struggle as the workout progresses and you look like someone rigging at the end of an 800 - reinforcing bad form instead of good form.
Daniels would also agree with the poster who said that the work at this pace also supports other paces as it makes them feel relatively slow.
That’s why you see Daniels emphasize this kind of work early in his training cycles - it allows for those running economy benefits to accrue during the longer workouts to follow and it makes the subsequent paces feel easier.
Them not being particularly stressful shouldn’t be overly surprising in that context, although if you stretch the reps out to 400m, you’ll see they become appreciably tougher.
All of this goes to 5k plans and above. The analysis would be different for middle distance runners.
I would note that there are those who would argue that being able to hold form while tired should be the real goal and this should could rest down. For this reason, and because most people would get bored with an 8 x 400 workout with 3-4 minute rests, I typically split the baby and do something like, 2 sets of 4 x 400 with 400 slow jog between reps and full recovery between sets.