This is a fairly general rule and you’d want to ignore if if your landscape shot had a particular point of interest in it that wasn’t on the third line. However if your landscape shot doesn’t have one specific point of interest it is probably a rule worth using.
My photographer friend went on to give a rather complicated reasoning for focusing upon this point a third of the way into an image that I don’t wish to repeat here for fear of losing many – however, in general if you focus too far into your image you’ll end up with objects in the distance nice and sharp but anything close to you noticeably out of focus. If you focus at the lower third you increase the depth of field in the foreground and as depth of field extends further behind a focal point than in front of it the distant objects will be reasonably sharp too.
Disclaimer – I should say that in researching this technique I found a lot of debate on the topic and a lot of technical language – in actual fact this ‘rule’ depends on many factors including the focal length of your lens, the format you’re shooting in (vertical or horizontal), the aperture you’re using and how far the scene extends away from you.