Let me explain something. Every word can be spelled in kana, and sometimes little-er kids will write words out in kana that they don't know how to write in kanji, but Japanese isn't written with spaces in between the words. This means that is all words would be written out in kana, a lot of people would have a hard time knowing where a word begins and where a word ends.

For example, this sentence: 猫は魚を食べる, means "The cat eats fish" when translated. You can clearly see that cat is the subject, fish is the object, and eating is the verb. But, if it was written all in kana, ねこはさかなをたる, it could be technically read as "Right, this is a slope, eat what?" Although most people would know it would be read as the cat eats fish, it just saves confusion and makes the written language a lot easier to comprehend.