These kind of warnings can also apply to rummage/garage sales. I finally put my foot down many years ago and refused to ever let my elderly mom have any ever again at her house.
She got robbed twice, and even foolishly let a woman into her house to use the bathroom, who stole stuff from her home while my mom was busy watching this woman's kid's outside, who were running all over the place. It was obvious in hindsight how it was a well orchestrated theft...and I'm sure these people have done it a lot.
She had a man and woman team work her over too. The woman was busy asking her questions and turning her while the man stole her cash box.
Also by letting strangers near/in your house, they can scope out things for future ill doings. They can usually find out if you live alone, if you have dogs, burglar alarms, etc.................as well as see if you would be an easy mark to come back and maybe even do worse than rob you.
Facebook photos can also be used against you. You'd be surprised how those seemingly innocent party pics, show bad people things they need to know. They can easily see things in the background they may want, as well as the layout of your home, windows, security, etc.