You use various chemicals in your work. Your supervisor's probably told you to read the label on a container before using what's in it. But if you've read the MSDS, why read the label? True, labels repeat what's in the MSDS. For example, a label may tell you what precautions to take when handling the material. However, the label will also tell you how to dispose of the very container you're holding in your hand.
Rock climbing is a sport that many people consider dangerous. To reduce the danger, safe climbers check their harness before each climb — even if they've already checked it multiple times. This redundant checking is one principle behind reading the label, even though you may have read the same information on the MSDS less than an hour ago.
What if a container doesn't have a label? Then don't use it! Even if you "know" what's in it, a mistake could prove fatal.