Premium-rate telephone numbers are used when inbound calls to businesses providing a specific high-value service over the phone require callers to be charged at a higher rate than that of normal numbers.
Because part of the call charge is paid to the service provider (you!), your business can be funded via the calls. A great example would be a technical support line that offers high value services that couldn't be billed any other way. In many cases, they are to cover their costs on a case by case basis rather than charging customers continually for a service they may never use.
Premium rate numbers are not just for companies wishing to generate revenue. They are used by charities for fundraising, providing a quick and simple method for donating money.