QR codes are often stuck over charging stations or vending machines, for example. However, they are sometimes also sent in letters. This is how people are tricked into entering personal information on fake websites or installing malicious apps. So check the target carefully.
QR codes should be designed in such a way that they are tamper-proof. For example, they should be placed behind a pane of glass to prevent them from being replaced. So be skeptical if they are simply stuck on.