Training to muscular failure increases mechanical tension per set, which serves as a compensatory mechanism to enhance muscle adaptation in low-volume training regimens but may impair adaptation when total training volume is already high.

Study: Low-Load Resistance Training to Volitional Failure Induces Muscle Hypertrophy Similar to Volume-Matched, Velocity Fatigue

Pushing muscles until they can't move anymore helps build muscle even with light weights and few sets—but this study shows it doesn’t do better than stopping just before total exhaustion if you do the same amount of work. So it helps when you’re doing little, but doesn’t mean it’s always better.

Study: Effects of resistance training performed to repetition failure or non-failure on muscular strength and hypertrophy: A systematic review and meta-analysis

This study found that lifting until you can't do another rep doesn't help you get stronger or bigger more than stopping before failure — and if you're already doing a lot of reps, going to failure might even hurt your progress.

Study: Acute effects of equated volume-load resistance training leading to muscular failure versus non-failure on neuromuscular performance

When people lift weights until they can't do another rep, their muscles get way more tired—even if they do the same total amount of work as someone who stops earlier. This means going to failure makes each set harder on your body, which helps if you're doing few sets, but hurts if you're already doing a lot.

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

Pushing muscles to failure doesn’t make them grow more than stopping just before failure — as long as you do the same total amount of work. So, going all-out isn’t needed, and might even make recovery harder.