What is the purpose of pasting stickers on glass?

Safety Measures for Glass Doors

Other than mere decoration, stickers on a large glass (a door or a large window) will indicate that the glass is there and help prevent anyone from walking or bumping into it. Under some lighting conditions, it is nearly impossible to see transparent window glass. This is one reason why sliding glass doors are now made of safety glass, highly tempered glass, or plexiglass.