Multiple lights mostly in my case. I have fixtures with multiple light bulbs in them so I want to turn those on/off as a unit. Also per room, a couple lights as mood-lighting, and all lights (to turn off after going to bed). If you only have one light per room groups aren't as necessary, unless you want to have an alexa-enabled group to just be able to say 'alexa turn on the lights' instead of having to specify said room or light, also useful for visitors.
I also have 2 servers that are not running 24/7 (manual backups mostly), those are grouped as well. They turn themselves off at night and alexa will then turn off the switches after that. When I want to wake them up I can do it as a group, they will turn on by themselves if power to the switch is restored. Grouping them is certainly not necessary, just a bit of convenience.