The Purpose of using Deadlifts and Olympic lifts in your training is to either become stronger or to become better at Deadlifting or Olympic Lifting.
If you want to increase strength, the most important thing to do is to increase the efficiency of your Central Nervous System so that you can express a greater amount of force with the body you have.
In order to increase your CNS efficiency you need to frequently train with weights that are 85% or more of your max.
The Russian Sports scientist Prilepin discovered that if an athlete deviates from a training volume that is ideal for certain percentages of their max, the effectiveness of their training diminishes.
Prilepin’s science shows us that if one person does power clean with 85% of their max for one or two high-reps sets till failure while another person trains with 85% of their max for sets of 2-4 with no more than 10 total lifts performed in a session, the person who performed the sets of 2-4 will benefit more from the training. Prilepin’s chart details the optimal number of reps per set one should make for a given Max % and how many total lifts should be performed in that session at that % for maximal strength gains.
If you perform 20 rep sets of Cleans, Snatches, or Deadlifts you will not be doing a good job of training your body to produce a large amount of physical force and you will be defeating the purpose of the exercise.
These lifts also involve a large amount of musculature and are very demanding, so fatigue sets in faster than it does for curls or other simpler exercises. When fatigue sets in, form eventually breaks down and the training effect is reduced due to poor form.
Strongmen perform higher rep deadlifts and overhead presses because the events they compete in are high rep deadlifts (car deadlift & tire flip) and high rep overhead presses (log press & axle clean). These guys also do a lot of low-rep work and take a lot of singles. Strongman competitions are, in large part, a test of strength-endurance. If your goal is to compete in strongman, you should do event specific training.
But I doubt many strongmen actually do ultra-high rep work. They’re not doing 50 rep squats or 50 rep deadlifts or anything. They also do their higher rep work when they’re training their events but they do their lower rep, strength training work when they’ve got the barbell in their hands and on their backs.