The Point Cache modifier lets you store modifier and sub-object animation to a disk file that records only changes in vertex positions, and then play back the animation using the information in the disk file instead of the modifier keyframes. This is useful when the computation required for vertex animation becomes so excessive that it causes animation playback to run slowly or drop frames.
Another use for this modifier is to apply the same animation to a number of objects, varying the Start Time and Strength settings for each so they don't all move identically.
The Point Cache modifier is also available in a world-space version, for which usage is the same.