•Why it earns a slot
Why it earns a slot: it is the book that put the word "Tommy" into permanent use for the British soldier, and it did it by writing entirely in his own barrack-room dialect rather than an officer's polished English.
Kipling puts the British Empire's common soldier at the center of Victorian poetry, writing entirely in the grunt's own dropped-h slang instead of an officer's or poet's voice.
This distillation is written from the freely available original, which is always the better read when you have the time: Project Gutenberg.