This is somewhat difficult to explain because it's just something that we do, sort of like it is a part of our birthright. I can definitely recall however, that it was some 20 years ago that I developed a true appreciation for spices because of its absence from the meal I had as someone's guest somewhere on an island facing the Caribbean sea. At that time I wasn't a great cook myself, but I knew a thing or two thanks to my mom. To be serious though, there a number of reasons I can think of as reasons why we use spices. Such a list would include usage for flavor and palatability, for preservation and color, for medicinal and health purposes.
Some people in general conversations do not necessarily make a distinction between herbs and spices. But herbs typically refers to the leafy part of the plant and would include items such as mint, thyme, cilantro. Spices on the other hand come from parts of the plant other than the leaves such as the bark and roots and would include items such as cinnamon and turmeric. In some cases, salt, which is a mineral is mistakenly added to the spice list, but it isn't.
I won't go into the list of spices and their flavors or how they add to the palatability of a dish here. The flavor and aroma profiles vary across the numerous spices available today - bitter, sweet, pungent, tangy, mild, lemony, etc. I've also noticed that spices aren't used in equal proportions even within the same country. While I'm not absolutely certain of the reason for the variance, I would speculate that some of these would include temperature differences in each region, population age, availability or scarcity, cost (saffron for example tends to be more expensive than many other spices), and so on.
For us, the main reason, if I were to give it much thought, why we use spices, would be that we just do. But most likely it is due to two primary influences, our heritage - East & West Indies, and our parents. These two influences have played a significant role in us being the foodies we are and for choosing to start Nature's Kitchen. Thanks for sharing with us.