You must use parentheses when combining multiple search terms using Boolean Operators because:
Databases process searches from left to right by default
Parentheses ensure your search terms are correctly combined. Search terms within parentheses are processed first. Then these search terms are combined with search terms outside the parentheses
Left Venn Diagram: Incorrect search due to no parentheses around similar search terms combined with OR
Right Venn Diagram: Correct search due to parentheses around similar search terms combined with OR