It is often hard to tell if the background of a photo is entirely white. It only becomes apparent when using the image alongside something else that is whiter, for example, in a predominantly white brochure highlighting presenters for an event or using a headshot in a website.
You cannot achieve an entirely white background by merely standing in front of a white wall or paper during a photo session. The photographer will need to use multiple studio lights to light the subject and the background separately. The lights' intensity can be adjusted independently, allowing the photographer to achieve an entirely white background. Alternatively, the person involved could be clipped from the image in post and place into a wholly white background digitally.
A perfect white background isn't required all the time, however. There are times when you want the image background to be off white instead.
In the photo above, we had edited the photo's background to be off white to stand out from the whiter book cover.