Also, you have to understand that a camera's meter is set to produce medium tones. So if your scene is darker or lighter than medium toned...you have to make corrections to compensate.
This is not much of an issue with color negative film because the photos can be corrected in the printing stage. If you are going to bracket with negative film, one or two full stops in either direction will make some, but not a lot of difference.
Slide film, however, is much more sensitive with it's exposure latitude. That is when adjusting by 1/3 of a stop will make a difference. Because when you shoot slide film, there is no adjustment when processing...what you shot is what you get. This is less forgiving but it is a good way to learn.