Data security policy creation

Summary

I need to understand how to create Data security policy

Content

I didn't get the context when we need to go for a data security policy creation. I understood the fundamentals of roles which compromises of functional security policies for UI access and Data Security policies for data access. But I have seen the security tab on items/item classes controls the access for items. So I didn't understand what are all the scenarios that require data security policies creation. Please explain with an example if possible. Also need to understand about the different sections involved while creating data security policies. Please help.