If it works for you, that’s great, but in most cases SmartThings actually will issue an individual command for each of the devices included in your group. It’s sufficient for many situations, but when there are multiple lights in the same space, such as the OP describes, it can still create the “popcorn effect” where there is a slight pause between the first light coming on and then the second light and then the third. If it’s very brief, That may not bother people, but sometimes it’s pretty obvious and can be annoying.
Zigbee does offer groupcasting as a way around this, where a single command is sent to multiple devices at once, but last time I checked, which was several months ago, this was not supported in SmartThings.
You’re absolutely right that you can definitely group lights together with the method you mentioned so that pressing a single switch turns on multiple lights, it’s just that you can’t force smartthings to issue a single command to the group. That’s probably just fine if you’re turning on, say, lights in several rooms at the same time. You likely won’t even notice if they are slightly out of sync. But when there are multiple lighting zones in one large room, it does tend to be more noticeable.
For those who are just looking for how to group lights in SmartThings, There’s a how to article in the community – created wiki that explains the various methods.
http://thingsthataresmart.wiki/index.php?title=How_to_Group_Lights_Together
But it sounded like the OP was looking for something more specific to groupcasting.
Previous community discussion which includes a video showing the popcorn effect in action for those who’ve never seen it: