It is about having a system without compromises.
In EF-M, they can make truly small, compact crop lenses. They couldn't be so small in RF, due to the much wider throat. The current lenses are essentially just as wide as the mount and if you never have helt one in your hands I highly recommend trying them. I was shocked to see how small they really are, compared to the impression I got from pictures.
Also, this way, we don't need to get into the messy boundary between high end APS-C and low end FF lenses, where equivalency makes for some redundant designs. They don't need to make a 17-55 mm 2.8 when you can just buy a 24-105 mm 4.0 L. With the RP showing that Canon can approach APS-C body pricing already, and future sales numbers likely reducing fixed cost per unit further, I find it easy to think of APS-C as something that is only needed for very compact cameras and lenses now. And the EF-M mount is just perfect for that.