Giving credit at every opportunity to the work of others in academic writing is essential to show how we develop arguments and viewpoints. Because we build on the work of others in order to form new knowledge, we must cite the work that came before us to help readers understand how we reached our conclusions.
Cartoon Via bLaugh
Not every statement in your writing needs to be cited. Every discipline has a generally accepted definition of what is considered "common knowledge" in that discipline, and those statements need not be attached to a citation.
This handy flowchart can help you decide.
Consult with your instructor or TA if you're uncertain of what would be considered common knowledge and what would need to be accompanied by a citation to a supporting source.