You might be surprised to learn that ancient Chinese has 9 tones, as Cantonese does now.Modern Chinese simplified it to 4.
Unlike they do in English, tones are very important symbols to distinguish different meanings in Chinese. Since Chinese words are normally within 4 characters and pronunciations are also limited, besides most of the Chinese characters come from pictographs, therefore there are very limited combinations in words pronunciations. If you don't distinguish them by some method like tones, it will be very hard to distinguish their various meanings, as many words can be pronounced the same. It's the significant feature of Sino-Tibetan languages.
For example, 肚子、独自、独资、犊子、赌资 are all pronounced "du zi", and the only thing to distinguish them is their tones. That's only a tip of the iceberg. You can imagine, if there is no such a thing like tone, daily conversations can be a total mess. For foreigners who live in China, they will find it very difficult for native Chinese to understand what they say even their pronunciation and grammar are both completely correct, only because they fail to pronounce accurate tones.