Well, there are seventeen types of Pokemon, each with their own particular advantages and disadvantages against other types. For example, a Fire Pokemon could easily defeat a Grass Pokemon but would have difficulty facing a Water Pokemon. Most Trainers tend to raise a team consisting of multiple types for that reason, but some only focus on one type in order to gain a more complete understanding of the nuances of that type.

I would be one of those people. My focus is the Ground type.