It seems to appear most on phpBB-based forums, or at least on the ones I've seen, along with the format you describe - just the numbers with no names. IIRC the list of names format is the default for a phpBB installation - I've broken mine so I can't check.
It was moderately interesting/useful - for example if I wanted to PM someone I could see there would be a reasonable chance of a quick reply.
Usually located at the bottom of a forum's index page, and showing member names and "guests" - generally meaning anonymous people who are not logged in - viewing that forum. I don't think WP publically showed guests but there are doubtless many more guests than members at any given time - I was one for weeks before joining.
Some even show bots, referring to the various minions sent out by Google, Bing, Yahoo and so on to scrape web content. From a moderator's viewpoint those were also shown on WP and easily exceeded any other category.
I've seen this on various forums but WP is the only forum I post in these days so I don't know if lots of other forums still do this or not. Can the mods here still see which members are currently logged in to WP
I'm sure it's a handy feature for mods to have even if you cant see which sections they have open. I sometimes have multiple sections open or I minimize my browser & get off computer for a while because I plan to go back to reading that section or finish my post later.