It takes time to build a botnet, but it can pack a lot of processing power. Bot herders use this processing power and anonymity that a bot network gives them for malicious acts that couldn’t be accomplished from a single device. Sometimes they sell or rent their network out to other people on the black market who don't have the technical skillset to build their own.
Distributed denial of service, or DDoS, attacks are one of the most common uses of a botnet. DDoS attacks leverage the large scale of a botnet to overload a network or server with a flood of requests. This can crash servers and block legitimate traffic. DDoS attacks are usually motivated by personal reasons, political reasons, and financial reasons when they are accompanied by a ransom.
It is hard to be an email spammer these days. But it is easier if you have hundreds or thousands of IP addresses that a botnet provides to send emails from. There are much fewer chances of getting blacklisted sending out spam.
Botnets have also stolen funds and credit card details. By infecting a network of computers with software to steal information, a bot herder can harvest the financial details of thousands of unsuspecting people in minutes.
The "mining" process that generates cryptocurrency like Bitcoin takes a lot of processing power. Sometimes it costs more in electricity to mine than the value of the resulting cryptocurrency. A bot herder can use a botnet to mine cryptocurrency for free.