Although the Japanese and Chinese languages differ considerably in their syntax and pronunciation, one characteristic they share, along with use of kanji, is lots of homonyms. Mathews' Chinese-English Dictionary, for example, lists 70 characters with the pronunciation shih (or shi, when transcribed in hanyu pinyin), which sounds like the English "shirt" minus the "t," varying only by their intonation.
In Japanese, many of those 70 characters are also pronounced "shi" (as in the English "she"). They include 史 (history), 師 (teacher), 市 (a market or city), 矢 (arrow), 士 (a warrior or gentleman), 使 (to use), 始 (first or begin) and 獅子 (shishi, lion), to name a few.
While the meaning of an individual kanji might be clear when written out, a way was needed to differentiate the words when spoken aloud due to the large number of homophones. This is done mainly by forming compound words, many of which are purposely redundant through the use of two characters having closely similar meanings.