You are learning about enzymes because they are vital to all life, not just those of humans. You may not choose to apply an enzyme to a task in your daily life, but that does not mean that they are not used. In fact, most reactions in your body probably use at least one enzyme. The role of an enzyme is to reduce the energy cost of a reaction, i.e. to make the reaction easier (also called "catalyzing" the reaction). This can greatly increase the rate at which those reactions occur and thereby allow cells to carry out their functions.

Enzymes work by binding to a specific molecule (or set of molecules) in a very particular way, such as bending it in such a way that a chemical group is more exposed and can more readily participate in chemical reactions. After the reaction is complete, the enzyme releases the remnants/products. The enzyme is unchanged by the reaction and can repeat the process with another molecule. Each enzyme is (more or less) specific and only binds to certain molecules, thus each only catalyzes one (or a small number) of reactions.

Some examples of enzymes at work are the breakdown of food molecules, producing the light in fireflies, and breaking down ATP to cause muscle contraction. For further reading, generally increasing in complexity, consider these sites
( C4K,
HowStuffWorks,
Khan,
RSC,
journals ).

[Some semi-related questions
about bromelain, an enzyme in pineapple juice.]