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Because a different tone can change the meaning of a word. For example, in Pinyin Mandarin Chinese-to-English transliteration which indicates different tones:
1. ài means (among other things) "love" as a noun or "to love, to be fond of" as a verb"
2. ǎi means (among other things) "short (i.e., not tall)"
3. ái means (among other things) "dull, dull-minded, simple, stupid" or "gnaw"
4. āi means (among other things) "sad, mournful, pitiful; pity" or "fine dust, dirt"

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I took all these meanings from Wiktionary. For other meanings, see:
ài: http://en.wiktionary.org/wiki/%C3%A0i
ǎi: http://en.wiktionary.org/wiki/%C7%8Ei
ái: http://en.wiktionary.org/wiki/%C3%A1i
āi: http://en.wiktionary.org/wiki/%C4%81i