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.