The majority of Leica rangefinder users tend to only own two modern new lens due to the high price of Leica glass, but many of us have a collection of older classic lens to complement our gear choice.
When choosing rangefinder lens it is very popular today to go for the fastest glass one can afford. My two fast glass lens are my Leica 50mm Summilux (f/1.4) and my Leica 35mm Summicron (f/2), both the fastest 50mm and 35mm lens I can afford.
Having such light strong lens to quote the term used by Thorsten Overgaard, allows you to use selective depth of field and the lens control light very well, you can often shoot direct into sunlight without risk of flare.
The problem with them is of course price as already mentioned and size, while small compared to modern SLR autofocus lens they are large for rangefinder lens and they often intrude into the viewfinder causing framing issues.
So today more and more photographers are choosing two lens at each focal length, e.g. a 35mm at f/1.4 or f/2 and a smaller cheaper version at say f/2.8 or slower. These lens are small, allow all the viewfinder to be seen and of course are much cheaper.