@Masquerade #1
I believe the Nc6 is simply the old Sicilian line, old as in history, and in ECO code it seems to be called Old Sicilian as well.
See e.g. the famous "immortal" game Alexander McDonnell vs Louis Charles Mahe De La Bourdonnais from their 1834 match :
http://www.chessgames.com/perl/chessgame?gid=1001165In that game the old fashioned way of playing with e7e5 and then Qf6 (or Qe7) is used. Later the Sveshnikov was invented as an improvement for black in that very e7e5 line. The old line (once played by Mikhail Tal by the way) is nowadays hardly played anymore. The Sveshnikov is relatively popular. I believe that GM Shirov still plays it, and even GM Carlsen (despite his positional grind down style) has done so.
Then at some point the Najdorf and the Dragon defense related to the d7d6 line became popular.
So, to summarize :
Black would play d7d6 to go for Najdorf defense, or some Dragon defense line, or Kalashnikov defense.
Black would play Nc6 to go for Sveshnikov, or the old lines with e.g. Richter-Rauzer for white, or the Lowenthal line (as in above mentioned match game), or some Dragon defense lines, or the Grivas (Qb6) lines.
As you can see the Dragon as choice overlaps a bit there, but still the reasons to play either d7d6 or Nc6 are completely different.