It sure does feel good, in the lats, too.
The myscles in the lower back take a beating just in the daily activity of maintaining posture while standing and walking. When you exercise them, it's kind of nice to give them a mini-vacation afterward by lightening the load.
There may be other reasons why people do it, but that's my rationale...
------------------
Never get pulled over with a baggie of whey protein on the passenger seat...