First of all, I have no plans to integrate thumbnail rebuilding into the EWWW IO plugin. There are several good plugins out there that do the task well, just search for regenerate thumbnails in the "plugin adder" and you'll find the top-rated plugin for this purpose.
Second of all, if you're considering a thumbnail rebuild, make sure you get to the end of this article, there's a special surprise for you there.
What this is really about is the fact that EWWW IO will re-optimize your thumbnails when you rebuild them.
I'm glad you asked, and the answer is that it is not possible to generate fully-compressed thumbnails using core WordPress functions. Mostly, it's a matter of speed vs. size in the compression process. In WordPress, the thumbnails generated are done as fast as possible, with little regard for the size of the image. Also, WordPress tries not to blow up your server by using too many resources when resizing images (memory & CPU usage can go nuts on larger images).
If you're using the free version of the plugin, this is all probably of little concern, but if you're using the API (and not on an "unlimited" plan), regenerating thumbnails could cost some serious coin as the process will overwrite all existing thumbs with new images.
The Special Secret Surprise
*If you're on an Unlimited plan, you can ignore this section, but you might need to email support to have your image quota bumped up (no charge).
For everyone that still pays per credit, I have good news and bad news. First, the bad news, it's still going to cost money, whichever plugin you use to regenerate the thumbnails. The good news is, if you got this far, you're eligible for a huge discount (66%). And here's how to get it:
Before you start regenerating thumbnails, check your account balance in the EWWW IO plugin status box. Write down your current usage. Then, when the regenerate process is complete, check the balance again. Send those two numbers to our support team, and ask for a "thumb regen" discount.
If you run into timeout issues during the thumbnail regeneration, try disabling EWWW IO temporarily, and then you can reactivate it later and just run a bulk optimize (keeping track of your usage, same as otherwise).
Of course, if you have any other questions about how EWWW IO interacts with thumbnail rebuilding plugins, or encounter any problems, let us know!