In my view, street photography primarily has one of two purposes, to document or to make art, even though these may both be present in some quantity. Artistic street photography associates less with the who, the where and the when, emphasising the composition such as lines, subject placement and colours. Documentary street photography associates more with those aforementioned human elements.
But these intentions behind street photography are easily brought to odds with the vulnerability present in the act. Making a photograph of a stranger walking down the street can, especially if you have a non-confrontational nature, cause anxiety and apprehension as to the consequences of such an act.
In most other genres, there is a certain amount of control in how you make a photograph, whether it's a portrait, a landscape or something else entirely. You can typically move and pose your subject, add or alter the lighting, adjust the set or props, or simply take your time.
On the other hand, what I find most satisfying about street or documentary photography is the lack of control. To make a great photograph of a unique moment with a stranger takes more than just technical skill. It requires a constant awareness of your surroundings, precise execution of timing and a certain lowering of your inhibitions, which is arguably one of the biggest components of the genre for me.