It is often hard to tell if the background of a photo is perfect white. It only becomes obvious when the photo is used alongside something else that is whiter. For example, in a predominantly white brochure highlighting presenters for an event or when a head-shot is used in a website.
Perfect white background cannot be achieved by simply standing in front of a white wall or paper during a photo session. Multiple studio lights are often used so as to be able to light the subject and the background separately. The intensity of the lights can be adjusted independently allowing the photographer to achieve a perfect white background. Alternatively, the subject could be clipped from the background in post and place into a perfect white background digitally.
This does not mean that perfect white background is required all the time. There are times when you want the background to be off white instead.
In the photo above, the background was edited to be off white so that it stands out from the book cover that was whiter.