I mean exactly what I said , submission to the forum websites like PHPBB, in the registration form you have some fields , one of them is an website address I thought it`s worth to say about this way of link obtaining. I reading here much about articles submissions, directory submissions, buying links etc. but not found anything about this way of link submission. I mean profile site of the forum you submitted, it can be indexed by google, if you have an your website address on the profile site you have one more link. I know there are many forum`s admins witch are set not indexing profile sites by google bot but there are many who don`t, think forum submission is a some of the ways everybody can use to gain more links.
like you said most of the profile sites are set not to crawl by search engines, if they are then you have a backlink for your site. Again you need to check if it is a dofollow link and page rank of the profile page is high then only you benefit.
You have right, I can imagine there are many ways of gaining links thru forum sites are much better, but this is one of the ways, I didn`t say it`s best I said about cos on my local area there are many forums on PHPLD script where owners allow crwawlers get all the sites on them ,including profile sites therefore cos that forum sites in many cases are builded only for links exchanging systems , everyone knows links exchange systems likes high site parameter, therefore in that exact example owners of that forums websites paid much attention to crwawlers index as many pages as possible to increase amount of points gained in links exchange systems.