It might be because you would want to hide the field as it is not relevant for users to see in the dashboard. By using HidePrefix of %, the dashboard might be hiding the field. Read about HidePrefix here:
It could be to easily identify what fields are actual keys you want to use. If you do not use the % symbol, nothing will happen as Qlikview uses associations. It is basically a matter of naming convention.
As stated above, it can also be used to hide those specific fields that you do not want to appear to the user