Presuming virus senders are able to break through any anti virus software or firewalls, what's

having them?

Presuming virus senders are able to break through any anti virus software or firewalls, what's the point inktm

Your initial assumption is wrong. End of discussion.

The only way they can ''break through'' is if the user downloads something and runs it. Most AV software would warn about the presence of a nasty when the download was finished.

Presuming virus senders are able to break through any anti virus software or firewalls, what's the point innortan

That's like saying - what's the point in having locks if a burglar can break a window?

The answer is that if you didn't have locks then anyone could walk in - they wouldn't have to be prepared to try and force their way in....
Prevention is better than a cure
with out a lock any one can walk through your home.. the same realtime example applies here too..

Even when you lock some viruses get inside the pc - The reason behind is the current antivirus programs works with two technologies ( Signature / Heuristic ) which is somewhere 10 years old technology,

In the case of Signature based Tech - the AV program will check for the signature if available in the database. Which means it can only identify the known viruses.

In the case of Heuristic Based Tech - The AV program will check the behaviour of the file. For sure to record the behaviour of the file it has to allow first virus to enter. if the first virus is power ful the ofcourse any antivirus for that matter can do very little to protect your computer.