When you open a savings account with a credit union, you become a part-owner or shareholder. Credit unions are not-for-profit and can pass revenue back to members in the form of a higher annual percentage yield (APY) on deposits, lower interest rates on loans, and no monthly maintenance fees on savings or checking accounts.
By contrast, brick-and-mortar banks and online banks are usually accountable to outside stockholders and may offer less favorable rates and terms. Also, note that credit union funds are insured by NCUA up to $250,000 whereas banks have FDIC insurance.