Usually providers allow to add additional IP addresses to your server. I don't understand why. I can't imagine any common usecase which needs several IP addresses for one server.
- Multiple SSL websites (not all browsers support SNI yet)
- Two or more applications that need to bind to a certain port
- Separation of different types of application traffic
- Multi-tenant servers, each of which have their own firewall rules
- Virtual IP, failover, CARP, VRRP, or heartbeat-type systems
- The server is acting as a router for one or more subnets
- The server is hosting one or more Virtual servers, each of which needs its own IP
- Per IP traffic accounting
- Different reverse DNS records
- Using outbound connections to services which have per IP reputation/access/rate limit control (eg. tcpwrappers, SMTP block lists)
There are countless more reasons that one might want to have more than one IP on a server, but these should at least get you started.