They are probably spambots - an automated program trawling the web looking for random forums to join so they can leave an url to a dodgy site, usually gambling or p*rn or similar. Funnily enough they don't always leave an url (I have no idea why)
I think the idea is (in part) search robots will find the web link on the forum and of course, the more times a site is linked to, the higher it will be in the search engine rankings.
We run a small phpBB forum and our policy is - if someone joins and doesn't post within 24-48 hours they are deleted and if they have a dodgy url, then they are deleted straight away, no argument.
There is a quick mod (for the phpBB forums at least) that the admin can do to stop the spambots registering, but it doesn't stop real people registering, we did it and it has worked for us, also a lot of the newer forum versions are implementing visual confirmation for new registrations (type in the characters you see displayed to confirm you are a real person) which also works.
Also, it's worth bearing in mind that some people join up expecting that registered members get some sort of extra content that is hidden to guests. When they don't find what they are looking for, they walk away.
Whatever the reasons, new members that don't add to the discussions are a waste of space