Op, your setup looks very uncomfortable from an unsupported shooting position. 3-4" eye relief from the exit pupil where your scope is mounted looks like it requires an excessive length of pull for a comfortable head position. Must be lots of strain in your primary arm.
As to your original question why the so-called gap shouldn't be bridged with two scope rings on the receiver and handguard, the answer is probably a bit nebulous. For the weekend bench or dirt shooter, it probably won't matter too much. A quality free float rail with a beefy mounting system, like Geissele's offerings, should minimize most issues.
But under high speed, you can see the different flex points in the AR, one of them being the barrel, and by extension the handguard. Tolerances stack and that movement under fire translates into a lot of flex and twisting of the optics and mounts/rings at different rates. Add other factors like a sling or a bipod and you're introducing more variables. I think for those who depend and/or fight with their firearms, it's one more potential trouble point that is eliminated by using a one-piece mount, when mounted directly to the upper receiver.