If training was truly random, we could see heavy deadlifts 5 days in a row or non stop 10Km runs.
With varied training, there is a plan behind it. Variance in our squat for example means moving between light/medium/heavy, back/front/overhead, wall balls, cleans, air squats, pistols, with lifting shoes or without, etc.
Fitness = Adaptability, so by varying the stimulus (exercise selection) you give your body more opportunities to adapt. This also means we’re healthier overall as our joints don’t get “stuck” in the same pattern.