tinyE: What does seeing Bruce's parents killed for the 48th time (is there anyone on this planet who doesn't know how they die!?), and that stupid Batman tied up nightmare have to do with setting up future DC spinoffs? :P
The Wayne murder is key to setting up the finale of the big fight with Supes. The two scenes mirror one another. Even if everyone knows the murder happened, you can't have that mirroring composition without the first scene, and it's detail have to be specific, it cant just relay on the general fact everyone knows that the wayne's were shot. It has to work like panels in a comic mirroring one another, like in Knightfall:
panel 1 panel 2 And the nightmare is all about setting up the invasion from Apokolips. You've got the parademons, Darkseid's trademark cannon fodder, you've got his giant Omega symbols burnt into the ground. There's more about Apokolips in deleted scenes, like
Luthor presumably in contact with Steppenwolf, Darkseid's right hand man. And then you've got Flash appearing with a warning from the future, presumably to do with the invasion. Or it could be something like
what happened to him in Crisis on Infinite Earths. Anyway, it will only make sense once we get to whenever he was from.