The clothes last longer, and some fabrics don't like machine washing and using a bag for more than a couple things in the washer and dryer makes it even more of a chore.
I've hand-washed before, and I still "hand wash" pillows in a bathtub. If I had a lot of vintage shirts or something that I really liked I'd probably hand wash them in the kitchen sink.
There is definitely some "more research is needed here". Some fabrics (like silk) are clearly marked "hand wash" but I wonder how much of it is backed by actual research.
Washing has several components, like detergent, temperature, agitation and time, and you have to balance them for good cleaning and minimal damage. Hand washing is usually weak on all counts, so maybe just using less aggressive wash cycles in a machine could work as well. In marketing, they tend to promote more aggressive cleaning just to be sure your clothes come out clean (it would be bad publicity if they didn't) at the cost of increased wear.
In addition, how much do cleaning affects how long your clothes last. I remember seeing an article saying that what damage clothes the most is wearing them, followed by washing, followed by drying. If true, there is not much you can do besides the most obvious and unsatisfying solution of not wearing the clothes you want to preserve.
Without better knowledge, I try to stick with the instructions on the label.
