[delluminatus]

The reason this is recommended is in order to improve the specificity of description: "he was very tired" does not tell us as much about him. If you just substituted "very tired" with "exhausted" everywhere, you wouldn't be adhering to the spirit of the substitution, which is all about adding detail. Maybe his manager hates him and he is leaving work, "browbeaten and resigned", or maybe he just didn't sleep last night and he is headed to work, "clouds in his mind and lead in his limbs" or whatever.