Once you have your data model defined with tables, you need to make sure the data in those tables follows some rules, is clean and consistent. Constraints are an excellent way to accomplish just that. If a constraint is set on a table, no one can insert bad data, no matter the privileges.
Constraints are not just making sure a value is an int or a string, they can be much more elaborate and useful.
Check out these guides that explain the power and versatility of constraints in depth