Re-encoding lossy into lossy, and what that costs
A JPEG has already had detail discarded. Encoding it as lossy WebP discards more, on top of an image that is already an approximation - and the second encoder is making its decisions based on artefacts from the first rather than on the original photograph. This is generation loss, and it is why repeatedly saving the same image through different lossy formats slowly destroys it.
In practice one conversion at a sensible quality is visually indistinguishable and substantially smaller, which is why the technique is worth using. Set the quality too aggressively low, though, and you are compounding two rounds of damage - so this is the conversion where it pays to watch the output rather than take the smallest number available.
Why photographs are where WebP earns its keep
Photographs are the heaviest thing on most pages and the thing WebP compresses best relative to JPEG. A gallery, a product listing, a blog header - converting those is typically the single largest reduction available in page weight, and unlike most optimisations it changes nothing about how the page behaves or looks.
The measurement that usually moves is the one tracking how long the largest visible element takes to arrive, which on a page with a photograph at the top is the photograph. Making that file meaningfully smaller improves the number directly, with no code change and no layout risk.
The instant feedback loop is the actual feature
Choosing a quality setting sensibly means trying one, looking at the size, trying another. When each attempt costs an upload, a queue and a download, nobody does that more than once - they take the first acceptable result and move on, which usually means a file larger than it needed to be or a setting more aggressive than it should have been.
When each attempt is instant because the work happens on your machine, you actually iterate, and you stop at the point where reducing quality further stops buying meaningful size. That converges on a better answer than any default, and it is only practical when there is no round trip between adjustments.