We're always looking for lenses that can perform magic. If you want to shoot in extremely low light situations or completely obliterate the background in your images, you need a lens (or two) that can do these with ease.
Shooting with a wide aperture allows you to drastically isolate your subject from the background and play around with bokeh, making it a neat technique for portrait photography. However, it also comes with a cost: the focus tends to be sharp only towards the center of your frame.
Street photography, much like landscapes, nature and wildlife is generally a solo hobby niche for photographers. You can go out on the streets and just enjoy snapping images and documenting the city around you.