SD,
If you are always behind a home or corporate hardware firewall, the need for a S/W F/W is questionable.
Ignorance is bliss.
The minute you have more than one device sitting behind your perimeter firewall, the more the reason to have a software firewall. Particularly for the lazy home users who won't have hardened or patched PCs.
Why ?
It helps delay (or in some cases, prevent) the spread of malware within your network which can spread by looking for open ports on other devices.
And anyway, both Mac and Windows come with software firewalls pre-installed, so you don't have to do any extra work or pay extra money apart from occasionally checking to making sure they are still configured properly and no unwanted ports are opened.
Finally, last but by no means least :
Do not forget that on "WiFi" you are not exactly "behind a firewall".