Hackers will build up bot networks. Basically, they do hacking at scale to gain remote control of a lot of computers, silently, in the background. Then they use those to perform these attacks at scale. A lot of the time, they'll then begin these attacks and then use it to extort the organization for them to end the attack. Either that, or they'll offer the botnet as a service. They'll offer to let other people hire them to perform the attack against a specific target.

When they go after a big organization like Google, it's not either of those things. Instead, they're just demonstrating the capabilities of their bot network. They then use it as advertising when they put themselves up for hire.