It does not add new frequencies above 20 kHz, but it does mean there is more brightness in the 10 - 20 kHz range. I find a clear and audible difference, particularly with reverbs applied to sounds with lots of high end. I find my tracks sound a lot brighter and crisper if I render at 88.2.
I think it may have to do with how Live calculates harmonics etc when rendering the reverbs.
Yes, it's because the processing of synths and FX produces less ALIASING
at higher sample rates. So the whole point of the discussion other people make; i.e. not being able to hear beyond 20KHz is besides the point. It's not about capturing more frequency's at higher sample rates. It's about getting less ALIASING
That's why some of the synths and plugins have internal oversampling. Try this; run an instance of Guitar Rig or better yet, Fabfilter Saturn to process a sound. First, set the sample rate of the soundcard at 44.1KHz in the Ableton menu and listen. Now set it to 88.2KHz and listen. Do you hear how much better it sounds?