Google is speed oriented organization and WebP is Google invented image format. I assume, that they just want that developers are more often happy to use these formats, especially WebP which is supported in more and more browsers. My personal opinion is that Google is thinking about the future of data, that all these images are using loads of data storage what needs to be saved somewhere. So, why not to recommend to use WebP format and in the same time promote it which means as more websites will start to use these formats, then more browsers will fully implement support for these formats as a standard of the web.
If you look in the future of the web, the data are taking more storage space by requiring larger storage infrastructure for servers etc. So new generating formats could help in the future to optimise potential issues in data storage infrastructure. As the data are taking more and more space in a web globally which will require more energy use, more servers, more storage etc. And all these potential issues with the growing value of data could be related to global warming as data storage could require more and more energy.
Recent research conducted by StorageCraft revealed the worries of IT
decision makers on these fronts. 43 percent of respondents said they
are struggling with data growth now and believe it is going to get
Organizations well outside the Fortune 500 realm that used to be
gigabyte scale data storage shops are now petabyte-scale storage
shops, facing challenges they never expected, said Brockett. More and
more businesses will need high-performing and scalable systems that
intelligently store and also protect massive quantities of data over
According to StorageCraft’s research, though, 51 percent of
respondents know they would benefit from more frequent data backup but
are unable to do so because their existing IT infrastructure doesn’t
allow it. More than half are not confident that their organizations’
IT infrastructures can perform instant data recovery in the event of a
Probably these data storage issues won't feel average website owner, but looking in a bigger scale globally, it's very important to promote and look for new ways to store and deliver data in formats which use less space. In this case, new formats for images are part of the potential solutions for data storage infrastructure requirements in the future.
I have a couple of websites which are fully supported with WebP and depending on a browser can serve prefered JPEG if required. The saving in real life could be even more than 70% based on my personal experience.