Looking at Cell Functions

All cells have a purpose. If they don't do anything productive, they are not needed anymore. In the big picture, a cell's purposeis much more important than acting as small organizational pieces. They had their purpose long before they started working together in groups and building more advancedorganisms. When alone, a cell's main purpose is to survive. 

Even if you were a single cell, you would have a purpose. You would have to survive. You would be moving around (probably in a liquid) and just trying to stay alive. You would have all of your pieces inside of you. If you were missing a piece you needed to survive, you would die. Scientists call those pieces organelles. Organelles are groups of complex molecules that help a cell survive. 

All Cells are not Created Equal

In the same way that cells survive in different ways; all cells have different types and amounts of organelles. The larger a cell becomes the more organelles it will need. It makes sense if you think about it. If you are a big cell, you will need to eat more than a little cell. You will also need to convert that food into energy. A larger cell would need to eat more and may wind up having moremitochondria to process that food into energy. 

While they might have a purpose, more advanced cells have a difficult time surviving on their own. A cell from your brain could not survive in a Petri dish for long. It doesn't have the right pieces to live on its own. It does have the ability to transmit electrical systems around your body. Anamoeba could survive in a dish forever, thrive, and reproduce. On the other hand, that amoeba will never help you transmit electrical impulses. The brain cell is far more advanced and has specific abilities and organelles. Simpler cells have a better chance of surviving on their own while complex cells can accomplish tasks that are more advanced.