basically i think it is so you don't have to go thru them one by one to delete them and the whole message isn't loaded so no possible viruses get passed on to your computer.

But this is just my general thought on spam filters - since I haven't used mailwasher myself.

you still can go to verizon and set up blocks - but usually the spammers don't use the same address or name to send the spam with - so it's hard to block all those when they keep changing them.

you can set up keywords to sort out the spam and block it or delete it right away with out you having to see it - just have it deleted or sent to trash folder.
Mailwasher might have settings for that - options or tools section?