What is the principal benefit an amateur telescope provides for viewing faint nebulae like the Lagoon Nebula?

Increased light-gathering power (via aperture)

The fundamental advantage provided by using an optical instrument, even a modest telescope, when observing faint deep-sky objects like nebulae is not magnification, but rather light-gathering power. A telescope functions by having a large aperture, which acts as a much bigger opening than the human pupil (typically 7 millimeters wide in the dark) to collect photons from the extremely diffuse light source. For example, an 8-inch telescope gathers light over 800 times more effectively than the unaided eye. This immense boost in light collection allows the observer to finally gather enough photons to reveal the object's structure and shape, which would otherwise remain invisible against the background darkness, even though the gathered light still registers predominantly via the rods, resulting in a grayscale view.