You can do a lot more than just sniffing.
1. You can perform ARP poisoning (Redirect a user to another website, eg malicious one)
2. You can hijack web browser sessions.
3. You can sniff sensitive information, like passwords if you use SSLStrip,
In a sense you are right, its more used for sniffing.
But you can collect a lot information about a particular device or user on a network.
And once you have this information, you can plan ahead and decide what kind of attach to unleash.