A rule of thumb in SAP is "Not to change the Standard Setting" Either it is Order document, item category,schedule line, pricing procedure, delivery type, billing type e.t.c.
Basically it depends on the BUSINESS REQUIREMENT,why we go for customization. 90% of the BR will not fit into SAP STANDARD setting. So, when u have to change any of the existing setting like creating a New sales order document for particular business only ( Foods business..) ( though we generally use TAN as standard item category ) we can derive a cusomtized one as per our needs.
BR needs may be:
1. New Partner Determination procedure
2. New Text determination procedure.
3. New pricing setup
4. New incomplete procedure.
there are a lot more to come. When your Business needs any change in the existing TAN setting, SAP gives u the flexibility of creating new Item category.
Also, if you start changing STANDARD setting ( TAN) as per your needs, SAP doesn't support it when u have some problems or any UPGRADE issue.
This is the real need of CUSTOMIZATION in any Business /CLIENT.
Hope this helps.