Hi Jorge,
On our craft gallery site our category pages are most important because the products under them change.
We use the category pages to hold common information. For instance, we have a category for handcrafted lazy susans. On that category page we detail care, alternate sizes available and how they are made.
On a different category page we may have a glossary of terms used on the products page. This avoids duplicate content on those product pages. It also sets the stage for linking from the product pages back to the common information. This ought to signal the importance of the category to google.
Best wishes
Handcrafter
