Your I-20 or DS-2019 proves that you are legally enrolled in a program of study in the United States with a certified educational institution. You should keep all copies of your I-20 along with your other immigration documents and should always have your most recent copy readily available.
You need this document as proof of your student status and use it (along with other documentation) in a wide variety of situations:
Your I-20 or DS-2019 must be signed by the appropriate official from International Student Affairs or the Office of International Admission.
The travel signature is valid for one calendar year for F-1 and J-1 undergraduate students (excluding Canadian and Bermudan students). For students on OPT or the STEM-Extension, as well as students from Canada and Bermuda, travel signatures are valid for six months.
Before you request an I-20, you should check to see if the one that you already have on your I-20 is valid. Any time ISA generates a new I-20, a new travel signature is added to the form.