“Help Yourself.” Means “Please Take Some”

When a host puts cake on the table and says, “Help yourself”, they are usually inviting you to serve yourself: “Please feel free to take some.” Help is not being used in its ordinary sense of assisting someone. The expression means you do not need to wait for the host to hand out each portion.

“Would you like some cake?” “Yes, please.” “Help yourself.” A natural response is “Thanks”, followed by taking a piece. “Help yourself to some cake” names the permitted item more explicitly. With “Help yourself” alone, the immediate setting supplies the limit; it does not grant permission to take anything in the house. The phrase is common in Australia but not uniquely Australian. Compared with “Make yourself at home”, it usually concerns one specific act of taking or serving rather than a broad invitation to relax.

https://dictionary.cambridge.org/dictionary/english/help
https://www.collinsdictionary.com/dictionary/english/help-oneself-to