NorthernSky,
(i) Because it is an easy way to talk about GPI or Ground Point of Impact, ie where that particular optical glide path intersects the runway.
(ii) It has a significant bearing on the TCH (Threshold Crossing Height) of your "dangly bits" - dangling is more proper for those who care.
(iii) They are all effects - the original question was constructed (I think) to avoid all of that.