Most reps are just a warm-up. It's the ones on the cusp of failure that matter most. The secret to growth is consistently failing in slightly more challenging ways.
One more rep. 5 more pounds. Slightly less rest. A more difficult body position.
This is called progressive overload. It’s the secret to all training. You gradually increase the stress (weight, volume, intensity) to challenge your body and encourage adaption.
There’s a famous story of wrestler, Milo, in ancient Greece who started lifting a newborn calf every day as it grew until he could lift a grown cow. His strength grew in line with the weight of the cow.
Rapid growth is attractive. Crazy challenges are appealing. Deadlift 400 pounds. Run a marathon. Do a handstand. These are all best achieved by getting 1% better. If rush to the finish line you’re taking on more stress than your body has adapted to handle.
On the other hand, you can exercise relentlessly without moving towards your goals if you don’t create progressive overload in whatever you are training. Just because a workout is hard doesn’t mean it provided a stimulus for growth.
Learn how to fail to learn how to grow.
Incorporating progressive overload can be as simple or as complicated as you want:
A common approach to Strength Training is to test your One Rep Max and then program workouts based on that. You could try to beat this one rep max every workout but the stress could lead to injury and there's little margin of error.
Instead, you can calculate a baseline weight for sets of more reps. For example, if your 1 rep max deadlift is 300 lbs, you might target 5 rep workouts at 255 lbs (85%) and 10 rep workouts at 225 (75%). You can then work to add weight over time at these rep counts, which in turn will increase your maximum capacity and eventually your 1 rep max.
But maybe all that calculating makes your head hurt. Let's get way way simpler:
You can experience the power of progressive overload by doing The Daily Pushup Challenge. Each day, do three sets of as many push ups as you can. You can do them back to back or spread them throughout the day. It doesn't matter. The only requirement is to do each one to complete failure (with good form).
No matter where you start, within weeks the number of push-ups you can do will be a multiple of where you started. If you do them together, the total amount you do will be less but the challenge of those last reps is still the same.
You're taking your body to the edge of its capacities and encouraging it to adapt through controlled failure. It's a beautiful and deeply satisfying process.