[With_Two_Flakes]

As Boboman316 said - the most important thing is to determine whether the ball crosses the LOS. He also said to help out the Referee on a bad snap. But you have to be careful in interpreting what he meant by that. I can't speak for the Mechanics in your area, but here in the UK we drill into our HL's that they can only help the Referee with their eyes, but not their feet.

A bad snap over the punters head that he has to scramble back to collect, before spinning around and kicking under pressure is precisely the type of situation where the punted ball will likely land a foot beyond the LOS, or maybe a foot behind it!! The HL must remain at the LOS to be able to determine this. If a K player grabs the ball and starts advancing it, only the HL is likely to know whether to allow it or blow it dead.

We also have the HL responsible for watching the block thrown by the "last ditch guy" ie the player who lines up a little in front of the punter to pick up any team R players who get past the O line. He is often a smallish player and may well be tempted to block low.