Yes, you are right, a management app seems very oldschool, but the thing is, most electricians order the device somewhere later in the project, but using the management app they can start building the project long before or even after golive without access to the device.
Some newer enduser features (like Sonos playlists or the simple when-then logics are configured directly on the homeserver.

When rebooting the HS it reads all flagged group adresses to get the current state. Also you can limit the amount of bus accesses from the homeserver in the configuration to not flood the bus. With a rather high bus rate the homeserver needs about 20 seconds here to get all information from the bus. This is done once per reboot. As the device seldom reboots (only after changing something in the programming) this is not a problem at all. All other reads and writes while running are direct without delay.