I do agree with you that it is not RoundCube's job... RoundCube is simply a client to whatever IMAP back end you point it at.
The reason I do the rest of that is that I have my own domain and wanted the experience of setting up and configuring Dovecot and Sendmail (Postfix is probaby a better way but I haven't had time to learn it quite yet). And, basically because I like to tinker. Setting up a simple forwarder in Sendmail wouldn't get me anywhere knowledge-wise, and defeats the point of setting up a web server in order to learn. I do e-mail support for a living and also used to teach Linux part time, so having hands on experience with my own setup is useful and educational when starting from scratch.
I also have RoundCube installed on that system, mostly for tinkering with mobile device support.
By the way, I'm not sure if English is your first language, but you're coming across as very rude.