The biggest practical advantage of a constant-aperture zoom is predictable exposure while zooming. That matters most for video, where a variable-aperture lens can make the image darken as you zoom. For stills, camera auto exposure can usually compensate shot to shot with shutter speed or ISO, so the benefit is often convenience rather than necessity.

It also helps in manual exposure, especially in low light or action shooting, where you may already be using the widest aperture and slowest acceptable shutter speed. With a constant-aperture lens, zooming doesn’t force an exposure change.

Tradeoffs: constant-aperture zooms are typically larger, heavier, and more expensive. Fast f/2.8 constant-aperture zooms are often part of a maker’s higher-end lineup, so they may also get better coatings, special elements, weather sealing, and overall build.

That said, constant aperture alone does not guarantee better sharpness than a variable-aperture zoom, and even good f/2.8 zooms are not automatically equal to primes. Primes still often offer wider apertures and may have optical advantages. A good f/2.8 zoom can replace several primes for flexibility, but not in every respect.