In a lot of the libgdx tutorials that I have took until now, I am told to resize the screen of the game using the SetToOrtho() of the orthographic camera. However, I don't get this part. Why do I have to resize the screen and make the sprites look bigger when I can just make the sprites themselves big at the first place?
You're not actually "resizing the screen" with
setToOrtho. You're building an orthographic projection matrix that takes into account the size of the screen (note the source).
By doing this, you effectively map the world coordinates of your sprites to pixel coordinates on the screen, which lets you work more naturally in terms of pixel positioning and pixel sizes.