Depending on size, such attacks can be very costly to organize, at least in opportunity cost (that is, using a botnet to attack BlueSky doesn't cost anything per se, but it does mean you can't use it for some other purpose, such as attacking someone else or mining Bitcoin).

If you're asking in general, DDoS attacks can absolutely serve a purpose - either to punish an organization that the attackers are unhappy with, or to hide some other more targeted attacks in a flood of errors, weird behaviors, and tired sysadmins.

One possible purpose is marketing. Owners of the botnet are merely demoing the capabilities for prospective customers.