Apparently, phone and laptop repair is a big thing in china. I see it as one of the main advertized points with many power supplies on aliexpress, suitable for repair shops.
And yes, 4.2V is what you need when you want to charge a standard phone LiPo cell. Some current limit would be nice too, but if you're not too worried about battery longevity, you can get by without it.
All laptops I've used in the past couple of years did indeed have 19V supplies I think.
It seems to me that the makers of power supplies such as this one expect to sell a lot more of them to small business than to hobbyists playing with 3.3V or 5V. Seems plausible too from what I've seen in Hong Kong; I don't think too many people there have the time, space or money to pick up electronics as a hobby.