The main point of vector files is that they can be the source of many different sizes of raster output.
Thus, realistic photos are almost always easier to represent with pixels than vectors. To create a really detailed image, you might need many thousands of complex paper cutouts, to the point where the image is much more mathematically complex than a grid of pixels, even millions of pixels. You might think of a set of vector shapes as paper cutouts placed on top of one another. So why might you start the process with one over the other if both usually end up at the same point? For raster, there are many image types that are just more efficiently represented as pixels than vectors. Vector and raster are most often two different points in the same process, rather than different processes entirely. Even if your image will always be viewed on screen as a PDF or an SVG or some other format, your computer monitor is a grid of pixels performing rasterisation. If you’re exporting a PNG or a JPEG, your vector image becomes raster at that point. Think of the end products of your work: if you are designing something for print, your vector image will be translated by the printer software into a grid that the printer itself represents in tiny ink dots. Vector software is really just another way of producing raster images. It’s not entirely fair because virtually everything you produce in software ends up as raster of some sort. This isn’t really a fair distinction, but it is the impression many people have when comparing the two image types. Here’s the same curve, the vector version on the left and the raster on the right: If you make the grid image much larger, you’ll end up with larger grid squares.
The point of this distinction between images defined by vectors and images defined by a grid is that you can make the vector image much larger and it’ll look just as detailed. There are roughly 50 pixels in each dimension, enough to describe basic shapes, but not much use for anything more detailed. In this case, the colour of each pixel varies from black to grey to white to give the illusion of a curve. This is the same curve as above, but the grid of pixels is much larger to make the construction more obvious: Within the grid, each square (or pixel) is assigned a colour value, and an image emerges from the arrangement of squares. “Raster” means something is defined by a grid.
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