New fileds & Existing fields in MS CRM

Hi,

I like to know about the fields in MS CRM, while customizing the forms in any entity as per the client request, its better required to use 'new_fields' or to use some existing fields??

Say, When I need to add a new text field in an contact entity, Is it required to add a new fields to CRM, than adding some unused fields in CRM?

If am adding an new fields to CRM, what is the advantage & disadvantage??

Is using the unused fields in CRM for  the requirement  will be causing some issues later?