The bot accounts are automatically recognized by phpBB based on their agent strings, and do not need to "login" like normal users must.
These accounts are used to ensure bots have "clean" content to index (e.g. no SID in the URL like a normal guest would have, no dead-end page links presented for things the bot doesn't have permission to perform), and also to control what language or even style the bot will see during their session on your board.
If and when a bot crawls your site and there is no matching bot account (which does happen frequently), they're receiving the same language and style as any guest user, and have the same access permissions as any guest user. And you simply may wish that the bots have different / more / less than that, and would create a matching bot account in order to control this.