As anyone can attest to, trying to squeeze out of a Tube doorway is not always comfortable. When there are dozens of other people trying to get out too, it's just not fun. Now, imagine that in addition to your polite and thoughtful fellow passengers, there are dozens of mindless, unconscious zombies trying to get out of that same doorway, dancing at the behest of an unseen puppet master. The result is that legitimate egress is denied, and you're left fuming. In less exotic terms, this is a DDOS through a botnet.
A DDOS (Distributed Denial of Service) is an effective way of shutting down a given service, such as a single server, wider web access, or even something as important as Pokémon Go. This is more than a mere annoyance, since it can seriously damage legitimate business: last year, the financial sector faced blackmail from the DD4BC gang, which demanded protection money in Bitcoin to avoid DDoS attacks (with one arrest and a detention earlier this year, that group might at least be stopped, but others remain). It's also been adopted as a weapon of activists fighting for and against various nations, using these attacks to take down opposing governments' online property.
One method of carrying out such an attack is through the use of so-called zombie computers in a botnet. These unfortunate machines have been compromised through malware. By day, their owners use them for mundane tasks such as doing the taxes and sending e-vites. By night, however, criminals use the additional resources which these machines give them to further their nefarious goals. Additional machines translate into more traffic, depleting available bandwith, memory or other resources. The formation of botnets is noteworthy, since they themselves form a criminal industry. In addition to the 'necromancers' who actively use malware to infect computers and turn them into zombies, there's a thriving industry of leasing out said machines to other criminals. These can be sold as whole botnets, in smaller batches, or even individually, with pricing relating to webcams and the gender of the computer owner.