Most of those bots try all adresses from the main DNS servers until they find one that replies to their formatting. That is no problem for the creators, because they aren't the ones paying for the bandwidth...
The spambot-nets are first looking for regular PCs to infect, and if they find one without current updates they infect it with a spambot-kit. Then that user will pay for the bandwidth when they send out spam-emails or link-posts on forums - probably cursing why his internet is suddenly slow without realising that his PC has just become a spambot. One of the ways those spambots distribute themselves is by placing infected downloads on pages claiming to provide you with free software or pirated software.
That results in the spampobt having only minimal costs - they aren't paying for the sending and bandwidth themselves, and that means everything they gain is pure profit. And there are two ways for them to get profit:
1) spam-advertisements like the one for gold farms - even if only one of 10000 targets plays that game and responds, that is profit to them.
2) cheap ranking-increase: if you find a forum post telling you nonsense around the links, even if you don't follow those links - google's bots (and the bots of all other search engines) will follow them and detect that link as an backlink to increase page rank of the advertised site. Spam several thousand of those posts in several hundred forums, and that site will gain front page in search engines for several days.
The second part is the reason why it's important to see if an SEO-offer for a website is from a legal SEO-working company or from a cheap botnet-spammer, because if you pay a botnetspammer for search engine optimization, you'll get only a few days of front page (just enough to be convinced of the work and pay for it), and after Google detects such an abuse your page will get downranked again (and perhaps even blocked from the engine) while the spammer vanished with your money...
