Having grasped the rudiments of Japanese grammar, what the student needs next are more words, more phrases, more turns of phrase. In short, more ways to get across the ideas that can be expressed so easily in English but for which the student simply doesn't have the requisite Japanese vocabulary.
One answer to this problem is found in affixes - nifty little prefixes and suffixes, written with a single kanji, that can be attached to ordinary words to create new ones. They function much like Latin-derived prefixes and suffixes do in English. Just as you can attach "anti-" to almost any noun to create a new one (e.g., anti-American), you can attach its Japanese equivalent in the same way (e.g., han-amerika). Once you know this, you can "anti" and han yourself left and right and, for the most part, be understood.
Thus, without going through the laborious process of slowly acquiring these useful affixes through many hours of reading, you can quickly build a larger vocabulary and expand your range of speech and your ability to comprehend. Even for those who have good many kanji under their belt, this is a great time saver. For those whose kanji is rather minimal or nonexistent, it provides a means of picking up words that would ordinarily be far beyond their reach, for they can learn these prefixes and suffixes as sounds.
In short, it is as though you only knew the word "simple" and then one day acquired "simplify," "simplistic," and "simpleton." And, perhaps more importantly, this approach allows you not only to learn words that are in the dictionaries but to actually create new words to suit what you want to say, just as you would do in English. The Japanese themselves are constantly doing this, and you have to know what is going on in order to keep up. Whether for your active or passive vocabulary, the use of kanji prefixes and suffixes is one area that should not be ignored.
There are 13 kanji prefixes and 50 kanji suffixes in this book. [via]