Most of the pages where the posts as well as sigs are, are not with the same PR as the index page of the forum. then what is the use of buying signatures for the links?
THey give powerful backlinks especially from high rank sites like DP. I dont buy them for the backlinks, i do it for traffic. A guy like me who posts a lot can get up to 1000 unique visits a month from DP.
what purpose a backlink from a signature will serve if it is from a post on a page whose PR is zero(but obvious the pages with post have PR less than the index page.)
People buy sig links for the purpose of getting their sites crawled more frequently and faster. It's also good for gettting traffic.