If you're asking for reasons for 'club' use (any airline, domestic or international)...
my reasons are:
1) place to dump my stuff (coats, bags etc) while I have a drink, a bite to eat, can read, watch tv, work on the computer etc in relative peace & quiet.
2) if the 'occaisional' problem occurs and your flight is delayed, cancelled or your're too early--its a place to 'wait'. Additionally if there is a problem with your flight, the crowds at the desk generally are minimal and thus the chances of a resolution to the problem is sooo much faster.
3) On international flights, I'll leave my stuff at the clubs, and do my duty free shopping, strolling, eating or people watching unencumbered.
4) And last but not least, its a great spot to meet up with other FT'ers