What is the difference between “…” and x“…” in an IF condition in a Windows batch file?

Question

I recently found the post Find if substring is in string (not in a file) where it is stated that considering

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Answer 1

The addition of x (or any other alphabetic character) in front of a string ensures that the relational statement is syntactically valid even when/if the string is empty.

Suppose str1 is an empty string. Then the comparison %str1:bcd=%==%str1% after the substitution degenerates to ==, which is syntactically invalid.

However, with an x in front, the comparison becomes x==x and can be evaluated. Naturally, adding the same prefix to each of the two strings does not affect their (in)equality.