Why? Currently, the volume of each room is not changed when it’s added into a group. If the volume is “right” for listening as a single room, why does it need changing because it’s in a group? 

Systems become ungrouped because of faulty software.

What has that got to do with “preset volumes” when selecting groups? The volume doesn’t change when rooms are added or dropped from groups. 
I’m not trying to be argumentative; just trying to understand the benefits of such a feature. 

> The volume doesn’t change when rooms are added or dropped from groups.

The most common scenario for device volumes becoming out of sync is a room being erroneously dropped from a group, then an alarm gets triggered. Alarms set a volume.

About half my mornings look like:

1. Alarm was previously set for a room with: “Options → Include Grouped Rooms = True”.
2. Some error causes rooms to be ungrouped.
3. Alarm plays, automatically setting the volume for the room and any remaining grouped rooms.
4. I log into the app, regroup the rooms, open the volume controls, and readjust the volumes.

> just trying to understand the benefits of such a feature.

You’re right, it’s a good point - this feature probably isn’t necessary if groups were more stable.
I have multiple alarms set, and I experience consistent un-grouping issues.

As said in your other thread, if your devices ungroup in other instances than after a restart, tak a look at your network.

That is unproductive and unhelpful.

For future readers of this thread experiencing the same issues, this is an excellent write up on the flaws of the current Sonos software.

My reading of how mDNS works is that it should suffer from similar problems to SSDP on routers that can't broadcast/multicast between radios, but it shouldn't offer new problems. But evidently it does: the huge amount of users who had perfectly working systems that suddenly had no devices found in the app shows that either mDNS is not a reliable discovery system on many home networks, or their code has some gigantic flaws. 

Future reader, if your system has recently become unstable, this is a possible reason.