  Why does the Zel'dovich approximation (ZA) work well to describe
gravitational collapse in the universe? This problem is examined by focusing on
its dependence on the dimensionality of the collapse. The ZA is known to be
exact for a one-dimensional collapse. We show that the ZA becomes progressively
more accurate in the order of three-, two-, and one-dimensional collapses.
Furthermore, using models for spheroidal collapse, we show that the ZA remains
accurate in all collapses, which become progressively lower dimensional with
the passage of time. That is, the ZA is accurate because the essence of the
gravitational collapse is incorporated in the ZA.
