Leica

Folks, i am completely ignorant here, so I wanted to ask...what Leica is being used for?

The way I understand it, Canon and Nikon is used for all types of professional photography. Medium format is used for studio and large prints. What is Leica being used for?

I think I saw 1-2 people with Leica before...doing some street shots mostly...i guess.

Also in Canon it is very easy to understand what set up you need in terms of lenses and bodies.

As an example, you can have all in one 5D mkiii and few L lenses to be able to do stunning images with top class IQ. How will this translate to leica?