E.g. run a query to get the average length of comments provided. LEN('') is 0 which reduces the average length; LEN(NULL) is NULL and is not included in the average. My interpretation of '' is that someone deliberately provided a zero-length comment, so this should reduce the average length; NULL values are not comments and should not be included when computing the average length of provided comments. Your interpretation of '' as "there is no comment" and NULL as "there is a comment but I don't know what it is" will give incorrect results, since all the rows where the comment is not applicable will be counted as zero and the average will be way too low.
This is an interesting comment. I remember on another thread debating whether NULLs should be allowed and used this AVG calculation example as one reason they should. In the case of my comment field, there is no need to ever make such a calculation as it would have no business value. But it's something of which to be conscious if such a calculation would have a business value.