"Drop voicing" How useful it is ? Sound, or Root movement or anything? I know the mean of drop2 and drop3 but I don't understand How useful it is?
For guitar, drop voicings are used to make chords more playable (because of the way the guitar is constructed compared to say a piano). On guitar, the use of Drop-2 chords is extremely common because of this. Drop voicings, from a more general perspective, open up a chord so it takes up more harmonic space and makes it less dense. For example, Drop-3 chords with the root in the bass give a very nice sound with generally a large gap at the bottom. For example, a Cmaj7 in drop-3 with root in the bass would be spelled C B E G from the bottom up. Notice the 7th from the C to the B at the bottom of the chord and the close minor 3rd of the E to the G, giving it a much more open sound compared to the standard C E G B. Drop 4 voicings are more rare in use because they just replicate the first chord with the bass note 1 octave lower (e.g., C E G B is still C E G B in drop-4, but the C is now an octave lower). Other combinations exist and are used (like Drop2+4, Drop2+3, etc) but they are more special use for things like voice leading or to achieve specific effects.
In short, the point of drop voicings is to give us other sounds and shapes of chords that follow an easy-to-use pattern. They also facilitate playing certain types of chords on guitar.