This was a topic I was just thinking about myself, so glad you asked it! There are a few ways I think about this from a “training system” approach.
If you approach training as wanting to recruit and stimulate the spectrum of your muscle fibers, then you would be leaving some fibers behind if you never ran faster than, say 5km pace. Hill repeats are a great method to recruit these extra fibers because they require more force to propel yourself forward, even though the pace isn’t as fast, and thus will recruit more fibers because of that. They also seem to not change your lactate curve by much, if any. Some will refer to this as non-specific lactate work. That is work at a high (relative to the individual runner) lactate reading but that doesn’t result in the deterioration of your lactate curve. Steve Magness has written about this and even produced data on it. I’d assume it’s still on his Science of Running blog.
If you’re more of a train the physiological system person, per Daniels, then you can simply think of it as a repetition workout to improve your economy and speed. Daniels doesn’t seem to talk much about bioenergetics and how the different workouts can influence your lactate curve, not that that’s the sole goal of training. He does emphasize that you train like an individual though.
How much and how regularly you do different sessions likely just depends on what distance you’re training for and your personal physiology. For 1500m, being able to generate and tolerate those higher levels is certainly a factor. For a marathoner, hills or a-lactic speed work works well, since the goal is largely to shift your LT curve as far out to the right as you can (as well as bring your AT as close to LT too), you don’t want to be countering that with workouts that would do the opposite.
I think it all comes back to individuality and balance! This is why most 1500m runners are not the best marathoners and visa versa. Each has a particular physiological make up that lends itself better to different race distances. I identifying that and then training to your specific needs is going to bring you as near your potential as possible.
Btw, sirpoc, I agree with the poster who said not to discredit yourself just because you don’t have a science background. You’re providing great insight into your training with relevant metrics, seeking to understand it, keeping an open mind, experimenting and adapting, and improving! That’s the scientific method right there, to me. When I got into running, after a year or two of reading the typical training books and articles, I made an effort to learn the physiology. While I did learn a lot in a technical sense, both biochemically and physiologically, I can honestly say that not a whole lot has changed as far as the structure I use and the workouts I run. Talk about a poor return on investment LOL.