Rasters are pixels with color values. You must have noticed when you scale up or zoom in to images, they 'tear' up - that is basically because the image is made of pixels and it is resolution-dependent
On the other hand, vectors are mathematical curves that are resolution-independent. Vector images can be manipulated at any size.
When you need to import images and graphics into a graphics program for design like logo, artwork, print etc, converting which to vectors gives better result. Some softwares have conversion algorithms but the best results are often when done manual operation which can be a tedious process.