Hi Juan,
I'll start with proper usage. You sell electronics. You have cameras that sell phones that have attributes like Screen Size, Disk Space, and Battery Size. You create these attributes, and associate them with an attribute set called Cell Phones. The store also sells headphones, with attributes like Bluetooth Enabled and Cord Length that are associated with a Headphones attribute set.
Every time you create a product, you can select the correct attribute set, and you'll be presented with the right attributes to fill out.
Attributes can be important for things like site search, faceted search/layered navigation/search filters, product comparison, and even promotional price rules and discounts. In essence, they help shoppers find the right products and learn more about them before they decide whether or not they'll make a purchase. They also help admins apply sales to select merchandise, among other things.
Now, let's say that you have an electronics store like Best Buy, and don't use different attribute sets, and you just associate all of the attributes with a default attribute set that you use for all products. With so many product categories, you're going to have tons of attributes. That's lots of fields that are loading. It's confusing for the admins, and not very efficient on the loading (including in the admin). It can make administering a catalog difficult.
So, if you only sell a few products, or only sell very similar products, you may not need multiple attribute sets. However, if you do have a more diversified store, it's a lot easier to start with a healthy well-planned list of attribute sets. Keep in mind that, as a product catalog grows, you can continue to add new attributes and attribute sets.
In all cases, it's hard to imagine being competitive in today's eCommerce landscape without full product data, which will often include a variety of attributes that shoppers can see on product display pages.
To add to this, you may need attributes for certain advanced product types, like Configurable Products. For example, if you sell a shirt in red, green, and blue, in sizes small, medium, large, and x-large, that's 12 simple products. Rather than having shoppers look at 12 separate product pages (a small blue shirt page, medium green shirt page, etc.) for each product, you'd only display one Configurable product page where the shoppers could select the color and size. In this case, Color and Size are attributes.
Long story short, your use of attributes is going to come down to your merchandizing, but it should be done with best practices to avoid needing to do a lot of extra work (or re-work) later.
Best of luck!
