Many of us have friends groups that we use to read our Friends pages. Many of us have filters for "Communities" or "People" or "Syndicated Journals", but it's a pain to keep them up-to-date. If you use them just for viewing, you can forgo the filters and use a new LiveJournal function.
All you have to do is add ?show=X to your Friends page URL. You can replace X with either P (for personal journals), C (for communities and shared journals), or Y (for syndicated journals). You can use any combination thereof, as well. So, for example, ?show=PY will show personal journals and syndicated journals, but not communities or shared journals. (You could also use ?show=PCY, but that would be redundant -- you'd view all of your Friends.)
Now, if you have another Friends group -- say, a mix of personal journals, communities, and syndicated journals -- you can apply ?show= in just the same way (http://www.livejournal.com/users/<username>/friends/<groupname>/?show=X). It will filter for all journals of the given type *in that group*.
The real reason why this feature was implemented, however, was to filter the friendsfriends page (http://www.livejournal.com/users/<username>/friendsfriends/). You can append filters to the friendsfriends page so that you can only see Friends of the Friends in a given group (http://www.livejournal.com/users/<username>/friendsfriends/<groupname>/). You can also use the ?show= option so you can only see your friendsfriends who are also of a specified journal type (http://www.livejournal.com/users/<username>/friendsfriends/?show=X). Or you can combine the two (http://www.livejournal.com/users/<username>/friendsfriends/<groupname>/?show=X).