From its name it was clear it was a trojan, you would use it if you need  to maintain/keep access, or sometimes get access using SE emails/forum posts etc. now these things are very easy to detect by most of the antivirus programs, and that's why there are loads of them, most of them try to emulate/reproduce the same functionality that was seen in BOF2k and sub7. it is also a way where the underground hackers can show thier programming skillz. one famous security URL that keeps track of these tojran Horse backdoors is http://www.megasecurity.org.

Now do not underestimate the power of modern trojans that have video logging capabilities, browser attaching, and firewall punching!!!  does Setiri ring a bell?!