Uses of a botnet

Botnets are exploited for various purposes, including denial-of-service attacks, creation or misuse of SMTP mail relays for spam (see Spambot), click fraud, spamdexing and the theft of application serial numbers, login IDs, and financial information such as credit card numbers

• Denial-of-service attack where multiple systems autonomously access a single Internet system or service in a way that appears legit, but much more frequently than normal use and cause the system to become busy.
• Adware exists to advertise some commercial entity actively and without the user's permission or awareness.
• Spyware is software which sends information to its creators about a user's activities.
• E-mail spam are e-mail messages disguised as messages from people, but are either advertising, annoying, or malicious in nature.
• Click fraud is the user's computer visiting websites without the user's awareness to create false web traffic for the purpose of personal or commercial gain.

Reference: Wikipedia: Botnet