Why do cells use two ATP at the beginning of glycolysis?

Glycolysis is a chain of reactions that results in the
conversion of glucose into high energy compounds ATP and
NADH.

There are ten steps that constitute the reactions
which go on during the generation of energy from glucose. The first five steps are
considered preparatory steps and involve the use of two ATP molecules. This converts
glucose into three carbon sugar phosphates.

In the
following 5 steps the products created after the initial steps are used, resulting in
the formation of 2 NADH molecules and 4 ATP molecules. Therefore there is a net output
of 2 ATP and 2 NADH molecules per molecule of glucose during
glycolysis.