In addition to making basic modifications to your tables, like adding and moving fields, you can make more advanced modifications that let you set rules for your data.
All of these changes can help make your tables even more useful.
Databases often include rules because they help ensure users enter the correct type of data. Computers aren't as smart as humans about certain things.
While you might recognize that two and 2 or NC and North Carolina are the same thing, Access will not and therefore won't group these things together.
When a validation rule is in place, it is impossible for a user to enter data that violates the rule. This would prevent users from typing something that wasn't actually a real state postal code.