Typically people do so with the intention of selling something or post adult content. Recently it has been getting worse with posts getting through that have no intention of being part of the site, just spammers.
If you think you have seen one and the staff has not seen it yet use the notify button to let tje staff know.
Google rank is another reason. The more links to a site that Google sees, the higher in the rankings it will be. If a site link is put in the signature file of numerous spambots then it gets a high Google ranking, meaning that people will be more likely to click on it when they search for something. Having people click on the actual spam links is also a bonus in it.
The makers of spambots are pretty lazy, or there are just so many people operating them, so they frequently use the same text or usernames, making them more easy to spot.
The thing I find strange is when they actually post something vaguely relevant to the topic that is not actually just a copy or rehash of someone else’s post. Gone are the days when they just post a load of text selling electronic goods etc.