There is no way to get a reliable reading for a fact type derived from a database column. Therefore, after import, you will generally be visiting each of the imported fact types to clean up the associated readings.
Selecting this option places a model validation error on each reading created during import, effectively giving you a task list for items to fix. Each error will go away the first time you change the corresponding reading.
If you don't want this errors, then don't select the option.
-Matt