The values are datetracked so that you can keep the history of the actual values you use. You can also set up rate values at a future date and be sure that this information will automatically take effect on the date you set, and not before.
The rates can be defined in monetary units, or as integers, numbers, days, or hours, in various formats. For example, you could define a rate in hours to specify the maximum number of overtime hours that can be worked per week.
When you define rates, an automatic process creates corresponding database items that formulas can access.
See: Dynamic Database Items