5 x 5 has been used for the olympics and athletes probably at least since the 1900’s and remember during this era people were impressed by feats of strength
Even Arnold Scwarznegger’s mentor Reg Park insisted he worked on these principles and he himself one MR Olympia previously as did Arnold.
If it’s strength, why would bodybuilders and athletes use it?
Well strength creates more of a numbing feeling, and stimulates the central nervous system which allows you when you start to train for more reps such as hypertrophy and endurance with greater strength, which in turn means more reps with heavier weight and… more reps with heavier weight means more muscle.
The other benefit of 5 x5 is that it gives a great structure for hypertrophy (muscle building) work meaning that you can maintain strength year round while building muscle, so for example you could do 3 days of hypertrophy work, 1 day or sprint or athletics, 1 day of core and 1 day of strength using 5 x5, this will enable you to have great posture, great musculature, athleticism and strength, which in turn equates to feeling fantastically healthy and vibrant.
The above layout is quite intense, a more balanced approach would be to do 1 of each style of workout if you wanted general health, i describe these to inform you of the options out there, and how much is out there. training should be fun, do find your style and what suits you, personally i like 5 x5 for olympic lifts and power lifts, so something like
Snatch 5 sets x 5 reps
Clean and jerk 5 sets x 5 reps
Wide grip deadlift 5 sets x 5 reps
Back squat 5 sets x 5 reps
You can include it for isolated movements to get strong e.g. lateral raises and bicep curls but that would be for function and strength not necessarily size and for hypertrophy you would move on to 8-12 reps with 3-4 sets and probably use supersets on the same body part I tend to like chest with back, shoulders with arms, legs with abs. Try it out!