When I started learning wing chun I had already trained in a few other martial arts, none of which had any particular emphasis of forms. I was completely unaware of the value these combinations of movements had in training. I knew enough to realize that the forms were for training and were not sets of moves to be used in fighting but I figured the movements must each have a specific use. So I did what I always did when I didn't understand something; I went and asked my sihing Harry.
In my initial training I was constantly pestering my sihing for more knowledge but with only one repeat question: "what is this movement for?" His answer was as concise and accurate as it was frustrating: coordination. I don't know whether he lacked the English to explain or if he saw some value in letting me learn and explore this answer for myself. In either case I am indebted to him for his oft repeated answer of one simple word.
The forms really are all about coordination. Teaching students the structures and movements patterns would be a tall order without the forms. Intelligently organizing them with progressing levels of difficulty allows students to learn and refine complex body mechanics piece by piece. Every movement in the form is teaching multiple movements at once and the various separate mechanics can be recombined in different ways into a slew of techniques. This compendium of movements gives you the flexibility and breadth in coordinated movement to combine into the fluid, reactive techniques of wing chun.
The forms also teach what you might call coordination concepts. Take siu lim tao as an example. It starts using one hand, progressing to two hands in unison, transitions between different structures using one hand, and ending with slightly more complex two handed movements. You increase the difficulty when you add in footwork, kicking, and more complex body coordination. The forms give a way to drill the movements, structures, and some of the concepts, with progressively increasing complexity. Simply throwing people into using both hands separately yet together, while kicking, and incorporated with footwork and power generation leads to them failing to do any of these correctly.
Probably one of the best things about the forms is that you can practice them on your own and, at least for the first three forms, with no equipment. This means a huge increase in efficiency of class time and more efficient use of your money. Students can and should practice the forms at home. Your sifu can correct your forms in class so you can go home and build coordination on your own thus spend class time learning to apply that coordination. Class time really is for spending time doing training you can't do alone like drills, sparring, and other reactive training.
Coordination is not just the conscious ability to use the various structures in wing chun. Wing chun's fighting methodology is highly dependent upon reflex. Training the forms repeatedly makes proper use of the structures much more unconscious when you begin to actually apply it to drill live, reactive skills.
Even the boxer or wrestler who spends time drilling to refine a movement for application is practicing a kind of form. Shadow boxing is a great example. The fact that it is not prearranged makes little difference.
So go train the forms and pay close attention to the details. They can be worth the effort.