Basically so that we can observe different phenomena and focus on the necessary details in different ways. When your eyes "saccade" - move rapidly from one point of focus to another, our brains are cutting out the information between the two points and just working to rapidly jump from one point of interest to another. With smooth pursuit, our brain and eyes are maintaining a tracking focus on slowly moving target so that we don't miss any detail - if the target speeds up though our eyes will switch to saccade movements to keep up with where the target is going along it's expected trajectory. The video below gives a brief introductory overview of eye movement and will familiarise you with some handy terms to research further.

https://youtu.be/FaC2RXBss2c