The notion of a "canon" plays a role in understanding how to make sense of a work of fiction, and its plot-line. As much as people poo-poo plot, if stories do not tie into a plot, a work will lose its focus and the audience will lose attention.

However, what's important is the notion, not authorial intent or official pronouncements by the owners of the franchise.

Sometimes the official "canon" books contradict themselves, obviously retcon stuff or try to cobble together a storyline out of independent plot lines that don't really belong together (the examples are so numerous its hard to pick out something). Sometimes, "unofficial" or "noncanon" sources tie together perfectly (Star Wars expanded universe supposedly does this), occasionally something declared unofficial or even denied as false can even be critical to understanding the major story itself. What is important is that we can form enough of a coherent picture to be able to enjoy several works of fiction as a overall narrative and not just as works that have nothing to do with each other.