You may choose to override the user’s permissions if you want to provide them additional permissions only when approving specific items or specific user’s entries.

Example:
Jason is a supervisor and has supervisor permissions already set up in his user profile. If you want Jason to approve all time for his staff, but also approve expenses for everyone in the company, you will need to set up a special approver type specifically for Jason to do expense approvals.
In this case, you will want to override the approver's existing permissions and set up permissions specific to the expense approval. You can achieve this by selecting the Override approver's existing permissions check box and selecting the appropriate permissions.
When you override Jason’s permissions, any approval paths using this special approver type will give Jason the permissions set up for that approver type (his regular permissions will still be in effect for other approval paths that use him as a supervisor).