Kodansha Handbook of Common Japanese Phrases
Words don't exist as independent units, to be inserted into grammatical slots by a speaker exercising absolute authority over their placement. Rather, they tend to come in neatly bound packages -- in fixed phrases or entire sentences -- ready to communicate an idea that cannot be expressed economically in any other way. For example, "I wouldn't do that if I were you" is basically a set expression, allowing for little change. If a student of English wanted to express the thought behind this phrase, but wasn't aware that a means of doing it already existed, he or she would be forced to resort to a great deal of circumlocution before the point was made.