A Portrait Lens will provide a flattering perspective of your subject that is difficult to achieve with an 18-55mm Kit Lens. Portrait Lenses have an increased focal length that slightly flattens the image, giving you a natural-looking photo that is closest to the way your eye perceives it.
Canon’s Portrait Lenses have a larger aperture in comparison to a standard Kit Lens. A larger aperture gives a more shallow depth of field, which in effect blurs the background of the photo. With this lens type, you can isolate your subject from a distracting background, effectively making them the star of your shot. If you want to reduce the blur a bit, all you have to do is close the aperture to the required amount, which gives large-aperture lenses a broader range of expressiveness over the Kit Lens.
Most of Canon’s Portrait Lenses have an Ultrasonic Motor that’s not available with the standard Kit Lens. This provides a faster, quieter and more precise autofocus function.