Dual mode for Fact table

Hi,

I know that you set dimension to dual mode if you have a fact table that is direct query and aggregate table that is import mode.  

If I only have one fact table that is direct query without any dimension tables, is there any reason to set this fact table as dual mode?  If I do set fact table as dual mode isn't that the same thing as setting it in import mode?  I saw a model that was designed like this and wasn't sure why it was designed this way as I didn't think this made any sense?

thanks!