The record substream option was made available for two main reasons - first one is it drops the CPU usage down substantially and when watching live as your tile of cameras (seeing all your cameras on the screen at once) are on on substream unless you make one full screen. Most people are aware of this reason.
The second reason is then for playback. If you watch all your cameras during playback in a tile fashion, prior to the substream option - many computers would max out 100% CPU trying to record mainstream live and do playback of a past event. Now playback is like watching live - when you see all your cameras on the screen, it will show the substream until you make a camera full screen. Knocks the CPU usage way down in this case.
It's nice he now adds the option if you want to record substream or not as there may be a camera or two that you don't want to (or you have a newer computer that can playback past events while also recording live and not max out CPU), but after you have tried to watch all of your cameras in playback at the same time, unless you have a screamer newer machine, you will certainly appreciate this aspect of the program.