Mailwasher views the e-mails on the server and you can delete them from there. Which means you do not download any unwanted (and virus riddled) e-mails onto your computer, so therefor your system will be more secure. But you DO have to do your bit in deleting these e-mails to make your system more secure.
The advantage of mailwasher is that you can delete any e-mails that you think are viruses or junk while they are still on the server. It won't stop any viruses because it cannot always identify them, that is what your virus checker does after you have downloaded to your computer and hopefully before you open them.
you can stop these things before they get to your machine
As woodchip rightly says, you do have to 'do your bit' by manually scanning down the list of incoming mail as seen by Mailwasher, but it takes only a few seconds, and I wouldn't be without it now.
The only slight gripe that I have is that if another email lands in your ISP's server as your authorised emails are being loaded to your machine after telling Mailwasher to process them (deleting the unwanted first), then it will arrive at your machine. However, (using Outlook Express here, btw), so long as you don't have the preview pane on OE open, these can be deleted immediately. (Happens rarely, just be aware).
I've had two emails with virus's over the last couple of days, the senders of which started with a christian name and ended with @ntlworld.com. They had headings like 'I just want a friend'. I've got Mailwasher and in one of the colums it said 'Virus' so I just deleted them from the server. That way they didn't touch my system. So yes, I'd say get Mailwasher. I've got the free version which is fine for me as I only have one email account.