Server Trojans are again the programs which when installed on victim's machine, then those machines can be used as base for attacking others. Victim's machine can be used as file server or proxy server without the knowledge of owner. Basically Hacker would be using your machine and bandwidth for attacking others. This can be very dangerous because in such cases if hacker is using victim's machine for attacking other machines, on tracing the attack, victim's IP will be shown, and victim may have to face legal issues in these cases.
Moreover if victim's machine is being used as ftp server, hacker can use it for downloading or uploading his own malicious code and can use it for supplying through victim's machine.