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I think anything that adds to your application would be a good reason to write one. That could be a new job that enhances your scientific skills, a big award, or new publication. The Weill Cornell website has some guidance:What do you consider good reason to? New job? or just publication ?
Send updates if something significant changes in your application (e.g. major award or publication). We are open to receiving substantive and occasional updates. Check your emails from us for other update instructions.