I would go so far as to say structure and form are probably the most important part of music. I envision music as a discourse, wherein an idea is expanded and brought towards a greater whole. That is a very cryptic thing to say, but allow me to elaborate. Let's start with an example, say you are listening to a piece of music, and you encounter a musical object (like a subject for a fugue, or a motive, or what have you, the determination of it as an object is what is important) for the first time. To every object there is a corresponding idea (or concept whichever you'd like to call it, I'll stick with idea here; additionally if you would like me to justify this assertion I can) of which it is an instance, but as we have only encountered this single object, we only know the idea to contain at least this. But then, upon repetition, in some way, the object is changed, and yet the fact that we identify it still as an instance of the same idea implies that the idea has grown to encompass more than what it once did. This is discourse in the sense that, when two people have a discourse, one contradicts (their assertions differ, vary) the other but through growth (for example, by learning more) they come to see their positions identified (they come to a consensus, maybe you disagree because of a misunderstanding and through the growth of alleviating that misunderstanding you are unified). This is I believe what makes music great, personally, hearing an idea grow and and take life in succession. To generalize a bit, contrasting sections perform this same process upon the entire piece, rather than a single musical object, where the idea of the piece encompasses what it once did not while retaining its identity.

We took something for granted in this example, which was the determination of the object. This is where form is crucial, because where there is form, there is an idea, and vice versa. It is the idea that takes the raw content and unifies it into an object. Ideas are abstractions over specifics (content) which leaves a form, which we may use to determine objects as instances of this idea. It is clear then that without form, the process I spoke of cannot take place, and that is of great importance to me at least since it is that process which I believe makes music enjoyable and great. Some conventions and limitations (form is always limitation) are necessary for coherence and communicability of your objects, and consequently, your discourse about them, a musical language (form) for encoding musical objects akin to how natural language is a form for encoding thought. Our language though is subject too to this process I described above, whereby the idea of it is expanded by way of development. As you said yourself, the great masters pushed the form and in this they expanded the idea, just how by developing a musical object we expand the idea of it and bring it towards a greater whole. The same question you ask yourself when developing a musical object is the one you must ask yourself when you are attempting to diverge while still maintaining coherence, it must not be the divergence of the *form* of the musical object, it must the development of the object, it must still be determined as an instance of the same idea for that idea to grow, if it is too distinct it will simply be an instance of an entirely different idea. Likewise we must not leave the form despite breaking it, in that the form will grow as it now encompasses something it did not. It's a hard line to balance, and it cannot in my opinion be reduced to a science.