MOBI is, and always has been, the format used by Kindle eBooks. (Amazon bought the creator of the format, MobiPocket, in 2005, shortly before launching Kindle.)
The prevalence of MOBI files today is mostly down to people ripping purchased Kindle books off their devices and releasing them to piracy.
The current file format for Kindle purchases, AZW, is just MOBI + DRM. If you strip out the DRM / customer fingerprint, you’re left with a MOBI file. So a large percentage of ripped books on e.g. LibGen will be in MOBI, even though the source was AZW. You can’t remove the identifying fingerprint of the purchaser without also removing the DRM entirely; so pirates do both.
The significance here is essentially that Kindle apps/devices will no longer willingly import+load+display pirated books originally ripped from the Kindle store. Because, at this point, that’s what 99.9% of MOBI files are.