Training closer to muscular failure increases muscle hypertrophy by enabling greater total volume and time under tension.

Study: Without Fail: Muscular Adaptations in Single Set Resistance Training Performed to Failure or with Repetitions-in-Reserve.

When people lift weights until they can't do another rep, they tend to grow muscles a little more than if they stop a couple reps short — even if they do just one set. This study found that pushing to failure led to slightly bigger muscles.

Study: Muscle time under tension during resistance exercise stimulates differential muscle protein sub‐fractional synthetic responses in men

When people lift weights slowly until they can't do another rep, their muscles make more of the proteins needed to grow bigger — even if they lift lighter weights. This study shows that pushing to failure boosts muscle-building signals more than doing the same amount of work quickly.

Study: Optimizing Strength and Hypertrophy: The Combined Effect of Intensity and Velocity Loss Thresholds in Bench Press Training.

When people lift weights until they’re almost too tired to finish another rep, their muscles grow more. This study found that training until you’re much closer to failure (measured by slowing down a lot) led to bigger muscle gains than stopping earlier.

Study: Similar muscle hypertrophy following eight weeks of resistance training to momentary muscular failure or with repetitions-in-reserve in resistance-trained individuals

This study found that lifting weights until you can't do another rep doesn't make your muscles grow more than stopping a couple reps before failure—even though the failure group felt more tired. Both groups ended up with the same muscle growth.