I equate Cisco SmartNet to Insurance. You have Health Insurance, Car Insurance, Home Insurance, Life Insurance, Business Insurance, Workman Comp Insurance...
Some are required by law, some to protect your home, your family and yourself from catastrophic situations.
SmartNet gives you access to Cisco Support, Code Upgrades (which includes new features and functionality) and hardware replacements if your box has a failure.
This equipment has a significant upfront investment, like a new car or home. If you insure these, you should probably insure your Cisco Gear with SmartNet.