Put simply, the load capacitance values are those the crystal manufacturers say are required to run their crystals at the specified frequency.
You need those values to ensure you get precise timing on you projects.
You can use different cap values perhaps given in circuit diagrams, but this may not result in the exact frequency being given, although it may not be noticeable in many circuits.
Have a look at any Microchip Pic chip manual - section Oscillator Configurations for a good practical explanation .
A good example is a 32k xtal running as a real time clock, if you fit say, a 15pf instead of the recommended 27pf you can find the clock will be out by several seconds per day.
HTH