The first function is creating tissue and cells. Each moment, our bodies are creating more cells to replace dead or dysfunctional cells.
For example, if you cut your finger, your body (if it's functioning properly) will begin – without even wasting a moment or asking your permission –the process of creating skin cells to clot the blood and start the healing process. This creation process is indeed a metabolic response, and is called anabolism.
On the other hand, there is the exact opposite activity taking place in other parts of the body. Instead of building cells and tissue through metabolism, the body is breaking down energy so that the body can do what it's supposed to do.
For example, as you aerobically exercise, your body temperature rises as your heart beat increases and remains with a certain range.
As this happens, your body requires more oxygen; and as such, your breathing increases as you intake more H2O. All of this, as you can imagine, requires additional energy.
After all, if your body couldn't adjust to this enhanced requirement for oxygen (both taking it in and getting rid of it in the form of carbon dioxide), you would collapse!
Presuming, of course, that you aren't overdoing it, your body will instead begin converting food (e.g. calories) into energy. And this process, as you know, is a metabolic process, and is called catabolism.
So as you can see, the metabolism is a constant process that takes care of two seemingly opposite function: anabolism that uses energy to create cells, and catabolism that breaks down cells to create energy.
Indeed, it's in this way that the metabolism earns its reputation as a harmonizer. It brings together these apparently conflicting functions, and does so in an optimal way that enables the body to create cells as needed, and break them down, again as needed.