WEBP is no longer "unusual" as it now finally has downstream support for the most popular use cases. The anti-WEBP memes are out of date and it's generally better than even a highly-compressed PNG.
One of the reasons people choose webp is file size. If your images are smaller then you transfer and store fewer bytes. That can have benefits on cost and speed of transferring those to the client.
Steve Jobs tried to do away with the filesystem in iOS. It should be very telling that there is now a Files app on iOS and iPadOS. Its improvements were also a highlight of recent releases, and apps are moving more toward using files instead of their own sandboxes.