Why NULL values are mapped as 0 in Fact tables?

Question

What is the reason that in measure fields in fact tables (dimensionally modeled data warehouses) NULL values are usually mapped as 0?

Answer 1

The main reason is that the database treats nulls differently from blanks or zeros, even though they look like blanks or zeros to the human eye.

Here is a link to an old design tip by Ralph Kimball on the same topic.

This blogpost talks about avoiding nulls in measures and gives a couple of suggestions.