I don't understand junk email filters

[KaBone]

Junk/spam email filters separate those emails from your regular emails. It puts them in a separate folder. What good does that do? Haven't they already made it to my computer?
I could create a rule to delete them, but that only puts them in the deleted folder which still puts them on my computer. Am I missing something?

 

[Trizoy]

Filter them at the source... before your client receives them.

 

[Matt_91]

You can probably set it to delete them within a few days of being in your deleted items or junk folder, so that you can check to make sure that nothing you want got put in there by mistake.
It just makes it easier, because you don't get clutter in your inbox.