The price matrix is used for overriding normal prices for rentals. You can make a special price for any item type (new, reg, etc.) for any or all days of the week, a certain date, and a particular time of day.
It will also let you override the days to rent, or let you choose the days to rent at rental time, and it will automatically bring in the appropriate price.
You can also create a price matrix for certain members only, so perhaps your employees get to rent at a discounted price.