can only speak for myself, but I generally like to browse through everything, there's always some interesting things popping up on magazines I haven't subscribed to yet. I think subscriptions is more like a personal feed and "all" is like browsing. and blocking stuff is- well, to block out stuff I'm really not interested in
I haven't tried it, but I assume it would show you only local content Kbin instead of including whatever users or content originated on another instance.
And then there is a block button in the right side panel, next to the Subscribe link. At least, for desktop...
For mobile (even in "desktop site" view), it seems to appear beneath the list of posts/etc rather than as a right side panel. Just use find-in-page and search for "Subscribe"; should be right next to that.
Yeah this is definitely needed. I am browsing all/active just so I can gather up some magazines I'm interested in but I'm hit with a ton of non-English mags/channels. I'm blocking them as I see them but there are a ton. It's all part of the growing pains for sure so I'm not going to be impatient about it or anything, but this is a needed feature.