Well again It's standard feature on phpBB3, most will have them displayed. If you look on who's online list on phpbb.com you'll see many of them:http://www.phpbb.com/community/
In the case of the bots and phpBB they are in a group like everyone else, they even have a profile but of course they will never access it. I set their preferences to hidden, I don't think they should be included at all but haven't gotten around to modifying so they are gone completely.
When a legitimate bot or anyone visits for that matter it/you send a user agent so the site owner can identify what they are using. For members the user agent would be the browser string. That can be helpful if for example you want to change things server side specifically to adress issues with that browser before the HTML is compilled and sent back to the browsers.
In the case of bots and phpBB you it's used to mask some aspects of the forum. Here's Googles cache:http://184.108.40.206/search?sourceid=na ... ads.com%2F
Some slight difference, for example the user names have been delinked, there's no login link etc. They don't need to be indexing those pages so the links are removed. Really irrelevant as I use robots.txt to prevent it anyway.
On side note you really need to be careful if you're building your own site as SE's frown on this. They call it "cloaking" and generally it would be used for less than legitimate purposes such as serving good content to the bot to get in the index and serving bad content to regular visitors.. I intend on removing it fully when I get around to it, actually was quite concerned about it to begin with.