@transponderings The primary reason I can think of is reach.
With hashtags, you can only see content if:
1. you are following that hashtag (in Mastodon -- by the way, following a hashtag was a very recent feature in Mastodon); or watching that word/keyword (in #Calckey)
(Also, if a post is "unlisted", it won't appear in hashtags even if there are hashtags, but will still be seen through groups.)
2. you will only see content that your instance is aware of
But with groups, a content is “boosted” by the groups account, which is then seen by anyone following it, even those not yet known by your instance.
For example, I created a new solo instance. If I post using only hashtags, since it is a new instance, my status update will only be seen in servers already aware of my instance. However, if I tag a federated group, it will be seen by everyone following that group.
Or, if I am a new user, my #ObservableFediverse is very small that it does indeed feel lonely and the system is broken (people have forgotten that they started similarly in other #SNS). I have to follow people, discover users from other servers, to grow my Observable Fediverse. However, if I follow one or two groups, I will immediately see status updates from everywhere.
PS
It's not new actually. Federated groups has been around since 2008 when the first #fediverse software was released (called #Laconica, which was renamed to #StatusNet, then renamed and known today as #GNUsocial). And #Mastodon finally approved adding its own groups feature. There are also #Friendica and #Hubzilla (both far older than Mastodon and still exists today), and #Streams with group features.
@Jessica