An input mask on a form "masks" what is typed in to the appropriate format. If the mask is on the table alone then when a user attempts to update and doesn't know the appropriate format they get an error.
In my opinion, if you want a specific format only, then put a mask at both locations. However, for dates I prefer to only mask the form. Reason being, I set the format on the table to be general date which includes a time stamp and on the form I mask it to short date. The user only inputs the short date but I also get a time stamp as well. This is useful later on for different reports, queries, etc..
Hope this helps.