Cells are the basic unit of the body of any organism.
Understanding how they work in various environments help to develope medicines or medical treatments. Knowing the process and how the cells of a determined part of the body work, you can develope medicine that will focus on that part and not have a general effect through all the body.
By knowing their basic needs, you can improve their functions by giving them the nutrients they need, this way, the organism will grow better. For example, if you want that a crop is more effective and the plant produces as much food as possible, knowing what it cells need will help out to improve the production.