At least relative to bodyweight.
I know that one is supposed to train not only in the low rep ranges when the goal is maximum strength, but to also squeeze in some 8-12 rep sets when going for strength relative to bodyweight. But why? Wouldn't hypertrophy be counter productive to that goal?
Or is there a case to be made for training exclusively in the 1 to 5 rep range?
If your goal is purely defined by 'relative strength' objectives, then gaining additional mass may not be that desirable. Sure.
Relative Strength = Amount of weight you can lift, relative to your bodyweight.
Absolute Strength = The maximum amount of weight you can lift, irrespective of bodyweight.
Generally speaking, smaller lifters do have higher relative strength. However, bigger lifters can lift more overall.
The key for most people (if they plan to compete) is to find their sweet spot based on their structure.
I'll never compete at 160 lbs, it just will never happen. I'm too tall and even at what surely looks skinny to most people I walk around at ~195 lbs. I'd also likely never compete at >240 lbs, so if I were to compete, I'd have to zero in on the weight class that permitted me to lift maximum loads relative to my competition. That could mean cutting, or it could be gaining some weight.
Most people seem to have an ideal size from which to express strength in strength based sports. If you're not competing, then the point this question is trying to make is rather moot. People do hypertrophy, because there is more to lifting than just strength, some people also want to look good.
I digress. Beyond that, accessory movements do generally help improve compound strength. This is why the majority of powerlifters (and weightlifters) do use accessory movements.
The accessory movements aren't necessarily 'for hypertrophy' so much as they are to support weak areas of the compound lift. Are you going to a 1RM arm curl, leg curl, leg extension or tricep extension?
Likely no, the loads and muscles involved are often too small to get much out of anything other than >5 reps for most accessory/isolation movements. It's the same reason a lot of rehab movements are in the 6-12 rep range (it isn't for hypertrophy reasons).
It may have nothing to do with 'hypertrophy' in a lot of instances. 8-12 isn't automatically a hypertrophy rep range, other factors need to be present too (like enough volume or enough stimulating repetitions, a surplus of energy, adequate recovery, etc...etc...).
The second consideration is recovery. You can't just do <5 rep training all the time, it beats up on your joints and body. More moderate and even sometimes high rep ranges are easier to recover from while still yielding a training effect. You have to deload, and simply using higher less intense, rep ranges are one of many ways to do that.
If you already are strong and have all the muscle mass you need then you can probably train only in the 1-5 rep range and become even stronger without increasing weight.
However keep in mind that skeletal muscle mass is only a fraction of your weight. Also a lot of this muscle mass is in your legs, especially if you are not used to strength training. So adding some extra muscle mass (to your upper body) may make you a lot stronger but only increase your total weight a bit.
Everyone have to find their sweet spot regarding weight. At 90 kg + maybe 10 kg excess fat I was weak. I could maybe bench press 60 kg. After a year of strength training I am now at 95 kg and can bench press 80 kg. Still not a lot, but I have long arms and slow muscle fibers. Anyway my weight (excluding excess body fat) have only gone up ca. 5 % but my strength is up more than 30 %.
Increased strength is the sum of two factors: increased muscle mass and better brain control of the muscles (neural adaption). I probably can increase my bench press to 100 kg without increasing my bodyweight only trough neural adaption by training with low reps. However I think this goal will be a lot easier and faster to achieve if I allow myself to increase my weight to 100 kg by training for both strength and hypertrophy.
EDIT
I overread the "relative to bodyweight" part. I don't have the time ATM, but will try to edit my answer accordingly later.
/EDIT
An individual's physical strength is determined by two factors; the cross-sectional area of muscle fibers recruited to generate force and the intensity of the recruitment. [Wikipedia]
By training in the low rep range, you cause the CNS to adapt to the stress. It will be able to recruit more muscle fibers and thereby exert a greater force, i.e. you have a greater strength.
Anyway, by training in the hypertrophy range, you increase the cross sectional area of the muscles and thereby increase the strength, too.
Steven Low suggests the following periodization of the mesocycle in his book Overcoming Gravity:
In the microcycle (1 week) he suggests two days of strength focused training and one day of hypertrophy focused training for strength focused weeks and vice versa for hypertrophy focused weeks.
