This is a good idea and I'm glad to see WP move forward with this. For most site owners and site visitors, this is an improvement.
The bandwidth savings can be substantial and it can result in a significant speed boost. For a given visual quality, a WebP image will generally be much smaller than a JPG, especially if you are trying for a high quality JPG. The other viable format for photos is AVIF, which is superior to WebP in the way WebP is superior to JPG other than max size (AVIF limited to 7,680 x 4,320 pixels) and browser support.
I've been serving up WebP images whenever possible for quite a while now. You can see significant improvements on GTMetrix and the like just by turning this on. In WordPress, this was formerly done with a plugin like Shortpixel (my prefered) or Smush or many others. All of those plugins are just API bridges to services that can be used for any site, not just one running on WordPress.
>>changed to WebP by default
That's not quite right. To get pedantic about it, I would say more precisely that in addition to all the sized JPG images that WordPress has always created when you upload an image, it will now also create sized WebP images to match.
Your JPG derivatives will still exist. They have to because browser support for WebP is not universal.
Your uploaded images will also be untouched, as always. The only exception to this is that with the default settings, WordPress will resize very large uploaded images, which is a good thing - clueless users love to eat up disk space by uploading 5000px wide images, so for sites with writers and editors who don't know about image sizing, that saves people from running out of disk space on their hosting plan.
Which brings us to the sticking point. The big debate in the WP community about WebP was not about whether or not it was a better format and would save bandwidth. That was clear.
The problem is that since some browsers still do not support WebP, WordPress can not "change" to WebP, but must add, WebP images. This means that effectively, it now creates twice as many image files and takes up almost twice as much storage. WordPress users with sites that have huge numbers of images rebelled. They were afraid that they would run out of disk space on cheap shared servers and such.
I maintain one site that has 9GB of images. We're already serving these as WebP so we already have the thumbnails, but imagine a site already at 9GB who is forced to turn on the WebP feature and finds they now have about 16GB of photos (rough guess, might be less).
The "premium" Wordpress hosts like WPEngine tend to be pretty parsimonious with their disk storage. At WPEngine, that particular scenario pushes you from the $30/mo plan to the $115/mo plan. At Kinsta it pushes you from $35 to $70.
Because of these concerns, the WP community had to create a way to turn off the WebP feature before rolling it into WP.
All in all, though, this is an excellent change and it opens the path for eventually adding other modern image formats like AVIF that could be turned on or off as users want.
So it is now on by default instead of the original proposal which was simply to have it on with no way to turn it off.