having a single lens, specially a prime helps you think about compositions, framing and subject and makes you concentrate on specific types of photograhy and helps you learn that you can't always get every shot. It simplifies the process which means you get a better focus (no pun intended) on the basics.
The downside is that you completely disregard everything to do with perspective. Zooming with your feet is not the same as changing focus lengths, and sometimes changing focus lengths is more important than simplifying achieving the same framing.
I combine the single lens exercise with a "10 shots in a day" rule to help me really think about the composition, subject, story, interest, mood, lighting. We all have a habit of forcing ourselves to take photos and going out snapping at anything and everything remotely interesting and come home with a load of photos which I will never edit, or will edit and never bother viewing again. I consider myself white good, often time I wont take a single photo, or I have 1or 2 from a days walk, but I still amass photos that I can't be bothered editing because there is simply nothing special about them. I would rather take 1 single outstanding photo in a year that get universal applause that I am extremely proud of than dozens of 'very good' photos. Of course if you do things commercially then the latter make more business sense. I appear to be in the latter group when I would prefer to be in the former.
Like the one lens rules this needs balancing with plenty of free exploration and trial and error. Its well regarded it takes 10,000 hours to become an expert at something so spending the ay shooting thousands of photos is not a bad learning experience as long is some level of thought, review and feedback goes on.