Why is EDTA used in protein purification?

EDTA (Ethylenediaminetetraacetic acid) is a chelating agent with a claw-like structure that is capable of sticking to and grabbing other molecules. By binding divalent metal ions such as magnesium and calcium, EDTA prevents degradation of DNA and RNA and inactivates nucleases that require metal ions. 

EDTA serves multiple purposes when used in protein purification - it eliminates contaminating divalent cations, hinders protease activity, and inactivates metal ion-requiring enzymes.