Leaving your PC or network exposed to the internet without any protection is similar to leaving your car unlocked.
The chances are you may have some valuable items in there and potentially some confidential documents which have been left in a briefcase. As a result, anybody can open the car door and access the interior and its contents. Sure, you could take the risk, but the law of averages dictates that at some point there will either be a break-in or theft of something you value.
This is similar with a PC or a network. Without a firewall it is relatively easy for an unauthorised person to access and steal valuable information, or potentially vandalise and damage your PC.