Distributions with dates in the name (YYYYMMDD_HHMMSS) are generated when I run a script because I feel there's a certain worthiness in the present state of the code.
*_nightly* are generated automatically whether I think it's a good idea or not. On the plus side, if I'm too busy developing to think about releasing, you can get my changes. On the minus side, there may be some unfinished feature or other loose ends. Generally I only commit code that is running and pretty much correct. It should always be an improvement.