In a more traditional modeling workflow we spend a lot of time adding, removing, and tweaking the vertices and edges that make up our model. Typically we do this via the various mesh modeling tools that we learned about in the Mesh Modeling Fundamentals course. These tools give us very good control but it also means there’s a lot of manual work, which can lead to more complicated mesh operations and ultimately, an unnecessarily complex mesh.
Modifiers give you the power to work in a less-destructive manner, with more flexibility and speed. To give you an idea of how they might be used, consider the Mirror modifier, which is used for adding symmetry to your mesh. Rather than recreating the same details on the left and right (or front and back, or top and bottom), of your model, instead you can create it on just one side and let the mirror modifier replicate it to the other side automatically.