Why use agarose gel in gel …

Typically, a DNA molecule is digested with restriction enzymes, and the agarose gel electrophoresis isused as a diagnostic tool to visualize the fragments. An electric current is used to move the DNA molecules across an agarose gel, which is a polysaccharide matrix that functions as a sort of sieve. 

The matrix helps "catch" the molecules as they are transported by the electric current.

Electricity is used to move DNA molecule fragments through the agarose gel.

This technique has lots of applications. Generally speaking you can analyze DNA fragments that result from an enzyme digestion of a larger piece of DNA to visualize the fragments and determine the sizes of the fragments.

In addition to its usefulness in research techniques, agarose gel electrophoresis is a common forensic technique and is used in DNA fingerprinting.