I was, naturally, curious about what it did and I couldn’t find any documentation so I did a bit of investigation of my own and asked a few people at Microsoft.
It turns out that it is primarily intended for validation purposes, so that if you know a column should never contain a null value and then, at a later date, a null value does appear in that column then you’ll get the following error when you try to refresh a table in Import mode:
Column ‘MyColumn’ in Table ‘TestTable’ contains blank values and this is not allowed for columns on the one side of a many-to-one relationship or for columns that are used as the primary key of a table
Read on for more information. It looks like it works just as you’d expect.