If you run a website, WebP is the easiest speed win available. The same picture at the same visible quality, at a fraction of the size.
Compare the JPG and WebP at the same viewing size. Savings vary by source and quality, and no particular percentage is promised.
Transparent areas become white in JPG. PNG preserves existing transparency but does not remove a JPG's background.
PNG ignores the quality slider. JPG and WebP encoding can change detail; this tool has no lossless-WebP switch.
Every current browser displays WebP. The risk is older software, some email programs and some upload forms, which still reject it. If a picture will be used somewhere you do not control, keep the JPG as well.
On a website you control, WebP is a good swap at the default quality of 85. Test the page after replacing the images, and compare the sizes shown by the tool, because the saving varies from picture to picture.
No. Changing encoding cannot reverse earlier compression or create missing pixels.
What can be opened depends on your browser. JPG, PNG and WebP always work, and a file of another type may need to be saved as one of those in another program first.
It usually is for the same visible quality, but not every time. The tool shows the size before and after, so you can see the saving for your picture.