In my opinion these sites take advantage of people who aren't regular users of the internet, often older people.
To use my mother, as an example again. She uses the internet most days to check email. She doesn't eBay - doesn't understand it, so avoids it, but regularly shops with trusted brands. She's used to searching for Marks and Spencer or John Lewis and clicking the top link on a page. She rarely googles for information because she doesn't know what she might find.
People like her are so conditioned to clicking the top link on a page that they don't realise it's an advert.
The government don't help by having a different look and feel to every official site, so you rarely get a feeling that an unofficial site doesn't look right.
When you fill in a paper passport form, do you read every single word on the form, and all the instructions, or leap straight in with the information? Having sat in a room with 12 other people filling in DBS forms recently, I observed that every other person in the room got stuck in with a pen and they all had to re submit their forms when they came back with errors.
These sites prey on the trust and good nature of others. They serve no useful purpose.
Don't get me wrong, I was furious with my mother - she's an avid Watchdog viewer and Daily Mail reader and must have read about these scans a million times, but the people who set up these sites know how people think and behave online and deliberately set out to profit from it.