Why is MLA formatting so important?
MLA formatting provides a consistent format for readers to easily comprehend.
MLA stands for "Modern Language Association" - the organization that created the formatting style which is most commonly used to cite sources within the humanities.
Every detail such as when to capitalize, when to italicize, what font and size to use, margin widths, and paper format are important because they are a set standard within the academic community. Correct usage of MLA demonstrates that you understand your reader's expectations, and are therefore a member of this community of academic writers.
What should be referenced:
If it isn't yours...reference it!