Why Kanji?
Some people may think that the system of using separate, discrete symbols instead of a sensible alphabet is overly complicated. In fact, it might not have been a good idea to adopt Chinese into Japanese since both languages are fundamentally different in many ways. But the purpose of this guide is not to debate how the language should work but to explain why you must learn Kanji in order to learn Japanese. And by this, I mean more than just saying, "That's how it's done so get over it!".
You may wonder why Japanese didn't switched from Chinese to romaji to do away with having to memorize so many characters. In fact, Korea adopted their own alphabet for Korean to greatly simplify their written language with great success. So why shouldn't it work for Japanese? I think anyone who has learned Japanese for a while can easily see why it won't work. At any one time, when you convert typed Hiragana into Kanji, you are presented with almost always at least two choices (two homophones) and sometimes even up to ten. (Try typing "kikan"). The limited number of set sounds in Japanese makes it hard to avoid homophones. Compare this to the Korean alphabet which has 14 consonants and 10 vowels. Any of the consonants can be matched to any of the vowels giving 140 sounds. In addition, a third and sometimes even fourth consonant can be attached to create a single letter. This gives over 1960 sounds that can be created theoretically. (The number of sounds that are actually used is actually much less but it's still much larger than Japanese.)
Since you want to read at a much faster rate than you talk, you need some visual cues to instantly tell you what each word is. You can use the shape of words in English to blaze through text because most words have different shapes. Try this little exercise: Hi, enve thgouh all teh wrods aer seplled icorrenctly, can you sltil udsternand me?" Korean does this too because it has enough characters to make words with distinct and different shapes. However, because the visual cues are not distinct as Kanji, spaces needed to be added to remove ambiguities. (This presents another problem of when and where to set spaces.)
With Kanji, we don't have to worry about spaces and much of the problem of homophones is mostly resolved. Without Kanji, even if spaces were to be added, the ambiguities and lack of visual cues would make Japanese text much more difficult to read.