RoundCube only runs when a user is accessing it, a mail server is always running. When a email that would be filtered comes in to a mail server in automatically makes the file in the right directory. Where as if RoundCube did the filtering it would be checking and moving more then one email. The main problem is the quantity of email the RoundCube would be processing at once, you could wright a script that would check and sort email ever hour but it wouldn't be as seamless as having the mail server do it.