Hey @Boesj!
In short, workouts like Bald, Whorl, Pettit reinforce consistency, add to your aerobic conditioning, and build beneficial volume on top of your scheduled intensity. Completing them regularly can help make you a faster, more well-rounded cyclist.
There’s a great blog post we did with lots of good info about what’s the point of relatively easy, hour-long endurance spins between harder days here:
The physiological effects on your body from high- and low-intensity training aren’t exactly the same, but studies show that when used strategically the two approaches can lead to comparable results for your real-world performance. This is why low-volume training plans include lots of intense workouts, which maximize the impact of less training time. Mid- and high-volume plans have more time to work with, but since your body can only tolerate so much hard training, that additional time is mostly occupied with the type of low-intensity endurance workouts this article is above focused on.
I see you’re in a High Volume plan, which has 3 HIIT days with 3 Endurance days in between. Quite a bit of volume already. 
While you may feel fresh to perform a Threshold workout on an Endurance day, if done consistently, over time you may build up unnecessary fatigue and potentially shift your training into an unproductive zone. However, if you are doing this every once in a while and consider it’s OK, then by all means go for it! Just be mindful of how this will affect future scheduled workouts and adjust if needed.
There’s a great feature we release a bit ago that’s called Red Light Green Light that you may find very helpful for these type of scenarios. This new feature will track your training fatigue, show you when you’re overdoing it, and offer smart adaptations to your training plan to prevent long-term fatigue.