Coffee can be divided into two categories, single origin and blends. With single-origin coffee, the beans come one source -- though this definition can be loosely interpreted, as the origin can be as specific as a single farm or as broad as a whole country. Single-origin coffee gives you the advantage of flavor and aroma purity. Unblended coffee is a true representation of its origin.
However, single-origin coffee may have its shortfalls. When done with care, blends can produce more complex beverages, with broader flavor and aroma spectrums. It is not uncommon to come across multi-origin Arabica blends as well as a mix of Arabica and Robusta. Blending Arabica is easy to understand, but why would a roaster introduce Robusta? The answer is comes down to preference. Robusta can help an Arabica blend better stand out in milk-based drinks as well as produce thicker crema for espresso.
At the end of the day, roasts and blends boil down to personal preference. There's no right or wrong, it's just a matter of taste.