The primary real-world benefit of a simultaneous dual-band router is that you can run devices that only support 2.4GHz at the same time as devices that can use 5GHz. Older routers that supported both speeds would have to step down to only using 2.4GHz if there was only one device on the network that required it.
But now I feel like I'm bashing Axiom Man. Please don't take it that way as it's not my intent. You just happen to hit the topic I do for a living.