Everything in nature is made up of particles. Dictionary.com defines particle as one of the extremely small constituents of matter, as an atom or nucleus.
There are several reasons for studying particles. The first is how particles interact in systems. As more particles are combined, they have a large surface area relative to their mass. They then take on different properties that have many practical applications, such as paints and coatings.
Also, when particles get smaller and smaller they start to develop interesting characteristics. The fundamental structure of the material changes, and we are able to get very dense and strong materials such as steel.
Finally, most materials can be worked with differently in particle form. For example, we can ground down stones into particles and recreate them into whatever we want. The same is true for pills as medicine is ground into tablets to make it easier and more convenient to take.