The spammers (and those that pay them to spam) usually couldn't care less about whether anyone here clicks on their links or not. They know that Google pushes sites up its listings based, in part, upon how many links from reputable websites (such as this one) Google's web crawling software finds for them. So the idea is to litter the web with loads of links, in the hope that Google will move a site from the 100th page of search results for 'technical college in Andhra Pradesh', for example, up to the first page. The spammers get paid for each link that remains on a site.
Some spammers will check back to see if their links are still here after a short while (in order to ensure that they get paid for them). If they regularly find that every single link they post has been zapped very quickly, they'll eventually give up trying to spam here. If, however, they were to find that all of their spam remained on this site, they'd then post many more such links. (i.e. getting rid of a few dozen spam links each day probably prevents a flood of several thousand spam posts every day). However some spammers clearly don't bother checking to see if their posts have stayed here and thus they continue to spam the site.