1) server host has to pay less for the bandwidth and storage, which are the biggest costs for them, end user has to download less, making it take less time and using less data, which is still relevant for mobile users, but some desktop users as well (rip). a 90% difference is massive (see https://squoosh.app/)
3) the end user doesn't notice the difference if they are just scrolling past an image or something
4) old formats are inefficient due to older tech.
5) webm (webp is based on that) vs h264 was a massive fight with licenses and different tech companies picked their side a decade ago. There is a reason apple can't play webms (they put their focus on h264), why you need to buy a hevc codec for 89 cents in the MS store and why you can't display webps on windows images. Because fuck you.
6) AVIF (and AV1) is the new hot kid on the block all the tech companies decided was _the_ new thing. No loicense or any of that shit. Guess what can be displayed in safari AND windows image viewer without a problem, despite being rather new? The sad side of this that anything that isn't AVIF is getting shit on, like JPEGXL, which seems a hype af tech which would allow another massive savings, is backwards compatible to jpg and still is fast (this is a problem WEBP and AVIF have, they are SLOOOW to encode, webp is still tolerable though. Using AV1 HW might change that but that's still rather new)