i'm bad in english, but still try to explain.
Product attributes are always set by the administrator, and customer can not change it. For example, if you set a colour as product attribute, you also have to define all possible variants of colors. There is no difference whether you are using conversion rates depending on the color value of the attribute or not.
Customizable products, however, allow the customer to select an attribute value depending on the preference. For example, if you sell T-shorts, you are able to allow customers choose an inscription on a T-shirt, without price changing. Customer may order T-short with any inscription that he wants to.
To be more precise, trying to explain the difference in terms of data structures. Product attributes add fields to commerce product entity, and customizable products add fields to line item entity. So there is the main difference with all the consequences.
Based on my experience:
I use a Customizable products to sell tea sold by weight. Customer allowed to enter any amount of tea in grams, and then i recalculate sell price using rules.