Have you ever found it impossible to express yourself in Spanish simply because you
didn't know how to conjugate a Spanish verb? You knew the vocabulary words that you needed to express yourself in Spanish. But you just couldn't remember how to conjugate the verb that you were using in Spanish. And without correctly conjugating the Spanish verb that you wanted to use -- the person who you were talking to had no idea what you were trying to say. And can you blame that person for not understanding you?
Imagine being on the phone and unable to conjugate the Spanish verb "llegar" or "to arrive." If you don't properly conjugate the verb how does the listener know if you want to say that you have arrived, will arrive, going to arrive -- or that you are arriving. Matter of fact, if you cannot properly conjugate verbs in Spanish the listener won't even understand who is "to arrive" -- you, he, she, it, we or they.
The trouble with Verb conjugation books is that they are not designed for "study." Verb conjugation books are designed to be used as "reference materials" -- not study or learning guides. Learning how to conjugate Spanish verbs with a Spanish verb conjugation book is about as fun as learning Spanish by reading a Spanish dictionary from cover-to-cover.
Not only is it not fun. It isn't a practical way to learn Spanish. Even for the most disciplined student of the Spanish language, it is going to be a real challenge to have to read a verb conjugation book from cover to cover.