Najdorf threefold repition In the Sicilian Najdorf there is a well known line which ends in threefold repition (if both players want).

[Event "NN"]
[ECO "B90"]
[Opening "Sicilian Defense: Najdorf Variation, English Attack, Anti-   English"]
[FEN ""]

1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 a6 6. Be3 Ng4 7. Bc1 Nf6 8. Be3 Ng4


Since white retreats the bishop to c1, the positions after 5. ... a6 and 7. Bc1 are almost the same (only difference is that the knight is on g4 now) and since it is black's turn, black gained a tempo (if not for the knight's position on g4).

Two questions on this line:


Why does black have to return the knight to f6? Aren't there any other useful moves that he could make and only retreat when being attacked?
Ignoring any match situation or Elo difference, etc, both players insisting on this repetition, does it mean that Be3 is the best 6th move in the Najdorf for white and does it mean that normal play with 6... e5 is very dangerous for black?
does it mean that Be3 is the best 6th move in the Najdorf for white
  and does it mean that normal play with 6... e5 is very dangerous for
  black?


"No" is the answer to the first question and "Not really but in any case, as hoacin points out, there are plenty of reasonable alternatives for black" is the answer to the second question.

Look, whether it is true or not, the vast numbers of cynical ordinary club players like me strongly suspect that every time a game like this was played it was preceded by the players meeting in the bar the night before and agreeing a draw and then picking their favourite "forced draw" variation.

When challenged the players will come out with standard BS along the lines of "Well, I spent hours preparing the X variation [say first 5 moves above] because I know my opponent plays that line a lot. All my preparation was based around move 6 and I expected he would play move y [obviously a different choice from the drawing variation] and when he didn't I had to fall back on the repetition line". 

It is complete hogwash. Everybody knows it. But if both players have decided, for perfectly good reasons [tournament situation, tired after previous long hard game, minor illness - cold, stomach bug, etc., opponent is a close relative] that a draw is what they want then there isn't much anybody can do about.