When we talk strength, we mean maximum explosive force for a short period, how much you can lift in a single or a few lifts. Your central nervous system should activate the body to the max and explode, bang! For this, the body must get used to extreme stress in the nervous system, mentally, in the muscles, and all ligaments and joints.
To build strength and to get your body used to extreme loads, you need to lift very near your maximum. The number of reps per set should be 2-6