It asks the device for it's capabilities. Then the driver can understand what (in this case) the outlet can do. Your example of the indicator light is a good one, but it could be other basic things that make the device work correctly.
Yes, it is a good habit to do so, and may be required in many cases to make things work.
They were supported at one time. The locks were so problematic, they were causing a support nightmare. They removed these from the compatible list, but did not remove the driver. In fact, I think there has even been an updated driver since it was removed from the official list.
There IS an built-in Schlage driver. It should detect it correctly. You probably don't want the generic driver.
You can sort of tell by the InClusters. If it had a 0X25 in the cluster it would be an older Z-wave device, with 0X5E it is a Z-wave Plus.
I usually google the DeviceID number and see if it comes up. The better way is probably to pull the switch and look at the numbers. The number on the face of the switch can often also help ID it, and that is pretty easy and safe as you just have to pull the decorative plate.
I suspect this IS a Enbrighten due to being a Z-wave plus, and that it supports S2 security.