k values affect how many rating points are awarder in the event.
High k means you'll earn a lot more (the equivalent of playing at multiple low k events)
IIRC players who get Top 4 at a World Championships qualify to the next, as do winners (or in some countries up to Top 4/8) of National Championships.
The highest rated players in a region also qualify, as do those who get in the top few at the grinder (last chance qualifier).