Non-extending zooms are mainly valued for handling and sealing. Because the barrel doesn’t pump in and out while zooming, they’re generally better suited to dust/moisture resistance and are less likely to act like a bellows that pulls contaminants into the lens. They also avoid zoom creep, keep a constant physical size, and often don’t rotate the front element, which makes polarizers, graduated filters, and petal hoods easier to use. For close work, they also won’t physically move toward the subject as you zoom.

Extending zooms do have tradeoffs in their favor: they can be more compact for storage when retracted, and may be simpler or cheaper to manufacture. A common downside is zoom creep, and extending barrels are typically harder to weather-seal well. Some designs may also rotate the front element.

A related but separate issue is extending during focusing rather than zooming. Lenses that extend to focus can better maintain focal length at close distances, while internal-focusing designs often show some focal-length reduction up close.

So, the main practical pros of non-extending zooms are sealing, resistance to creep, and easier filter use; the main cons are that they may be larger all the time and not as compact to store.