I find these confusing too.... maybe they are targeted at people shooting video. As f2.8 lenses, they should be fairly compact,(but it looks as if they are both 58mm filter size, so appear to be about the same size as or a little smaller than the current 28/1.8 USM). Other than those reasons (video? compact?), I just don't see any reason for them. Maybe Canon will explain when they do press releases.
Most likely I'll keep using my 28/1.8 and wishing there was a modern version of the old FD "New" 24mm f/2.0. I would have really welcomed a USM EF version of that lens! It would probably be too close to the 24/1.4L, though, for Canon to consider offering one.
It's odd to see an ultrasonic lens without a gold stripe, too.
One other thought, I think Canon was taking a hard look at creating an EVIL (mirrorless) camera.... As a registered user and CPS member I got a survey from them about EVILs, probably about 5 or 6 months ago. I know quite a few other folks here mentioned getting that survey, too. Maybe rather than establishing a distinct mounting system for that they are looking at more compact primes in the EF mount that can be used on both EVIL and the DSLR line. I've often thought that there might be a market for a more pro-oriented Rebel/xxxD series, too (similar to what the 7D is for the xxD line)... perhaps instead of an EVIL.... or it might actually be an EVIL.