Does the <li> tag always have to be nested within the <ul> tag?

Hi! Why does it always have to be nested, or what would go wrong if it isn’t? I played around with the code and previews in the Elements and Structure exercise in the Introduction to HTML lectures, and figured that UL means that a list is coming up and indents the succeeding text, while LI makes a bullet point for the text. By this, if you were to create an unordered list that you wanted sitting flush with the rest of the text rather than indented, you can make a list with the LI tags, and omit the UL tag, it seems? With this, why would it be bad if we didn’t list LI tag inside of the UL tag?
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