The reason for a DDoS botnet is usually either the wish to have
enough firepower to virtually shoot people/sites/organizations off
the net, or extortion, or a combination of both. The extortion of
gambling websites before big sports events is just one example of
many cases of extortion involving DDoS. The attacker usually takes
the website down for a couple of hours to demonstrate his ability to
do so whenever it pleases him, and sends a mail to the owner of the
website, asking for money to keep the firepower away from his site.
This sort of business model is well known for millenia, and merely
found new applications online.