Most cell cultures can be stored for many years, if not indefinitely, at temperatures below –130°C. ATCC has recovered cells from cultures cryopreserved for more than 40 years.  The advantages of cryopreservation far outweigh the required investment in equipment and reagents. These advantages include:
Generating safety stocks to prevent losing a culture from equipment failures or contamination by microorganisms or other cell lines.
Saving time, energy, and materials that would be required to maintain cultures not in use.
Preserving cells with finite population doublings that will ultimately senesce.
Preventing phenotypic drift arising from genetic instability and/or selective pressure.
Creating a standard reagent that can be used for a series of experiments (source: ATCC guide for Animal Cell Culture).