Restriction digest is most commonly used as part of the process of the molecular cloning of DNA fragment into a vector (such as a cloning vector or an expression vector).  The vector typically contains a multiple cloning site where many restriction site may be found, and a foreign piece of DNA may be inserted into the vector by first cutting the restriction sites in the vector as well the DNA fragment, followed by ligation of the DNA fragment into the vector.
Restriction digests are also necessary for performing any of the following analytical techniques:
*RFLP – Restriction fragment length polymorphism
*AFLP – Amplified fragment length polymorphism
*STRP – Short tandem repeat polymorphism