Why are cells important to biology?
Water and separation
They separate this water from that water....the inside water from the outside water....extracellular matrix from cytosol..and so on. This separation of water leads to electrochemical potential across the membrane that can drive (is the source for driving) a number of important reactions for the cell.
I'd say simplistically, that cells are important for compartmentalizing and separating inside/outside.
I'd say because they study the simplest life that exists...
An alternative answer as opposed to Dave's, I'd like to add on maybe some basic facts on cells.
Cells are truly the basic units of life, and so studying them will give us an insight on what life became to be, why it came to that way, and it can also give us an idea on how they interact with one another, and maybe even prove the theory of evolution.
Our body has trillions of cells, each will a different function. If scientists study those cells, we would have a better idea of how our human body works together with cells to let us live a healthy life.