Re: Can someone explain "drop D" or other tuning down scenarios?
Well, first you have to pick apart two different kinds of tunings, you're mixing them all up.
The first is "Drop" tunings, where you only detune the low E string (sometimes more, but usually just the E). For example, Drop D refers to when you tune down your low E down a whole step to a D. The point of a drop tuning is either to make lower notes more vibrant, allow drone (low D/A/D) to ring out, or to help with different chordal configurations (you can barre the lower three bass strings and play a melody with your remaining fingers on the higher strings). It's actually a lot of fun, and can be used anywhere from neo-classical to metal to blues.
Next, there are "Open" tunings. This is where you detune more than one string so that when you strum all of the strings, you get a chord. One of the more famous ones is, from lowest to highest strings, D/A/D/F#/A/D. Here, you can often barre all of the strings on one fret to change the pitch of the chord. Also, once again, playing melodies can be modified on the higher strings, as the guitar is not set to tuning in fourths (except B in standard tuning, don't worry about it).
Last, there is half-step down. This is where all strings are flatted by 1/2 step, keeping the guitar tuned to fourths (and a third going to B) while allowing different pitches. This can completely change the mood of a song. It's really there for you to use at your disposal in certain cases; if it sounds better, use it.
Everything is used to one's discretion, wherever they feel it sounds better.
The point of all these different tunings is to allow a MUCH wider sonic pallete with which to musically create the sound you want. In other words, it gives you more options.