There are numerous reasons why photo consent is important. Generally, photos taken in public spaces do not require permission, but it's still a good idea to ask. Photos taken in a private setting require consent for legal reasons. You cannot simply post private images of someone without their permission. Additionally, there's an issue of respect and safety. Everyone has the right to refuse to be a photo subject or not to have their images on display. However, it's more serious in some cases, such as when a person moves to a new city to avoid a dangerous stalker. Since a basic google image search could locate that person in seconds and put their lives in danger, asking for permission could save lives.