Diction, also known as word choice, is the focus of special attention by good writers. Novelists and poets in particular rely on their word choice to convey their meaning as clearly and powerfully as possible.  

Consider this excerpt from  "The Darkling Thrush"  written by Thomas Hardy:  

"So little cause for carolings /Of such ecstatic sound /Was written on terrestrial things/Afar or nigh around,/That I could think there trembled through/ His happy good-night air/Some blessed Hope, whereof he knew/ And I was unaware."   

Let's briefy  analyze Hardy's word choice in his poem. What did Hardy mean by "little cause for carolings"? No doubt he was hinting at some unhappy circumstance.  

What other choices were there for his chosen word "trembled"?  He might have also chosen the words shaken, fluttered, wavered, or quaked; they're all synonyms for trembled, but none seem so clearly and powerfully to supply Hardy's meaning.  

Hardy's work perfectly exemplifies the importance of diction. Certain word choices engage the readers with a clearer and more potent message. So, for the new generation of writers, the watch-word is diction.