If you assign approval responsibilities to a specific user or role, no further logic or criteria can be put into that assignment. The only criteria available for the approval step are the step entry criteria. By associating rules at the assignee step via approval rules, you can dynamically assign approvers based on the same object record information used for the entry criteria.

• Dimensions Dimensions enable you to map fields from a variety of business objects including, but not limited to, Opportunity, Quote/Proposal, and Agreement Term Exception.
• Approval Rules Approval rules are used to associate Salesforce business objects, approval rule dimensions, and entry criteria to determine which assignees are required during the approval workflow.