The simple answer is:
- Anything you can't do or wouldn't want to try in a real helicopter
- Anything which is cheaper/quicker/more efficient to train in the sim than in a real helicopter
So, a sim particularly lends itself to training emergency procedures, as well as CRM, MCC, and to provide a challenging and realistic environment for line checks etc.
As stated above, these do not necessarily require investing in an FFS, which does not provide a huge amount of extra capability for the money.