Magnified Live View can be very useful for achieving accurate manual focus. The confusing part is that people often mean the preview itself may not look especially sharp when viewed at extreme magnification.

Two main reasons:

1. You’re viewing at a very high magnification. At that level, tiny blur, noise, and other imperfections become obvious, even if they would never matter in a normal-sized print or screen view.
2. Live View is a fast preview, not the final processed image. The camera typically has to convert sensor data on the fly, so the preview may use simpler processing and less sharpening than the final image.

So “pixel peeping” during focusing is still a good technique for getting the sharpest focus possible on a static subject. It’s just that the magnified Live View image is not necessarily a perfect representation of how the final photo will look. In general, pixel peeping can also exaggerate flaws that are insignificant for real-world viewing.