Expense rate is used if the expense amount being raised depends on another factor.
If you are entering an expense code for mileage, the rate might be $.80 per mile (or kilometer). Here, the amount being raised by the user depends on the miles (or kilometer) that he has traveled. So when the user selects this particular expense he will be prompted to enter the Unit [Number of Miles (or kilometer) that he has traveled], and the system will automatically calculate the expense to be raised based on the rate you define while setting up the Expense Rate.