One of the reports from the Korean air flight Boeing 777 that crashed in San Francisco states the instrument landing system was inoperative at the time. If it were fully functional would it have made a difference? Absolutely. The ILS, or Instrument Landing System, provides runway centerline guidance and glideslope angle guidance to within half a degree of accuracy all the way down the approach to a safe landing. If it were operational, and if it were used, the pilot would not have rifted so low on the approach without having to disregard the ILS glideslope display entirely. Watch this video to understand - the glideslope appears as the pilot sees it in the top left of the screen, and the profile view at the bottom shows how it works from a side view.
Without the ILS the pilot had to rely on visual clues. Approaching on a beautiful clear day over water creates two unexpected problems, the first that it is easy for even experienced pilots to look around instead of focusing on the runway, especially since they are watching for other traffic during visual VFR conditions. The second problem is the approach over smooth clear water. An over-water visual approach is hazardous because it offers few clues of height and motion. If the pilot were approaching over a city or even over a forest or field, there would be natural and human objects that would appear to move, and grow larger as the plane descended, offering critical visual clues. An overwater approach does not offer these clues. One report states the pilot had only 43 hours experience in a 777 and this was his first landing. Given the conditions mentioned and his low experience, the landing should have been handled by the more experienced co-pilot.
Condolences to the victims and their families and a wish for speedy recovery for the rest of the passengers.