«زَيْدٌ رَجُلٌ عَالِمٌ»
Zayd is a learned man.
«رجل» denotes a broad category; «عالم» adds the feature of learning. Instead of merely being told that Zayd is a man, the hearer receives a more informative characterization.
ExpressionAdded information
زَيْدٌ رَجُلٌBeing a man
زَيْدٌ رَجُلٌ عَالِمٌLearning as well as being a man
Not every adjective merely narrows a category. If the quality is already familiar to the listener, mentioning it may instead serve emphasis, praise or clarification. Context establishes its particular benefit.