If you're talking about why kanjis are in the language, this is the history of Japan with China, you might want to read some literature about the subject.
If you're asking about the utility of using kanji, there is a couple. It's separating words and grammar in a broad sense (because words don't consist exclusively of kanjis). Also, the same word can have different kanjis giving different variations of nuances. Finally, each symbol holds a certain meaning so, in some sense, you can understand the text without knowing how to pronounce it. There is also inconvenients, mostly that you need to learn them all. This is generally known as Logograms, if you want to study this in more details.
Sadly the different readings for the same kanji are an extra layer of complexity that we just cannot avoid, it's part of the language and how it evolved. Writing in hiragana is not "wrong" because you'll be understood, but it doesn't look serious so it will depend on the context. It would be suited for a casual things like mangas, chatting, learning, etc. But even then, you'll rarely see text completely in hiragana. For journals, documents, business and literature, kanjis are mandatory.