I don't understand a line about HTML elements mentioned in MDN

From https://developer.mozilla.org/en-US/docs/Learn/Getting_started_with_the_web/HTML_basics

Note: Simple attribute values that don't contain ASCII whitespace (or any of the characters " ' ` = < > ) can remain unquoted, but it is recommended that you quote all attribute values, as it makes the code more consistent and understandable.

Above is the line stated about HTML elements and I don't understand it. Please explain with an example.