Mike Johnston has just republished on article of his from a few years back called "Why 40 mm?" Since I've been using a 40 almost exclusively for the past year, this was of particular interest to me. I wish I could say there was a grand plan behind my use of the 40, but honestly the main reason I bought one is that it was cheap. I found an f/2 40 mm Summicron-C online for about $400. The f/2 35 mm Summicron with virtually identical optics sells for about $1500 used. Given those options I bought the 40 and I've been quite happy with it.
What I like about the 40 is that it feels malleable. In some cases it can feel long, like a 50:
In other situations it feels wider angle, like a 35:
The flexibility is almost like using a zoom, but with one lens.
Years ago I did in fact use a zoom. I imagined I was giving myself the ability to react to everything. If something was close I could zoom wide angle. If something was far away I could telephoto in to crop close. The end result was that my photos had no consistency. They had the look of being taken by a variety of people. This is why Philip Perkis calls zooms "the work of the devil."
The first time I went to a fixed lens was quite a leap. What if there's a far away scene and I can't zoom in? What then? It took a little while but I got used to just one lens. Less is more in this case, i.e. less variables to mediate my worldview. After all, my eyes don't zoom in and out. Why should any other part of me do it?