Arm sleeves are usually made of neoprene, and are intended to provide gentle, consistent compression in order to maintain ideal blood flow in the arms. The logic is that if your arms remain warm, they will perform more consistently during your game. This is especially important in sports such as basketball and football that require precise, controlled arm movements throughout the entire game. Whether or not arm sleeves are effective in this manner is open to debate. Another valid reason to wear arm sleeves is to provide comfort on cold training days when an undershirt is simply too bulky.