Quickbooks desktop is a powerful tool when things are working well but the program's ability to solve your accounting needs can be constrained by the data you have.  There are a number of reasons why you may want to start a brand new file for your companies data. some of those are: 

QuickBooks runs slowly; upgrading to Enterprise may not solve this
Data Damage; all efforts to correct have failed
List Limits; All lists 14,500 (individual limits of each list is 10,500)
Changing your EIN; changing from Sole Proprietorship to a Corporation
Stop/Start tracking inventory; You cannot change inventory items to non-inventory part items
File Size; QuickBooks are optimized for files of a certain size. Larger QuickBooks company files are prone to more incidents of data damage and performance issues.

 Pro:10mb-50mb;    Premier:50mb-300mb;    Enterprise:3000mb and up

Creating a new QuickBooks desktop company file:

This video walks thru everything you need to consider and things you need to do in creating a new QuickBooks data file The slides for this presentation are available  here.  If you are interested in the TBX tool mentioned in the training video, you can get that here