Leading strand and lagging strand

I know that the DNA is replicated in a 5´ > 3´ direction.. However, I don't understand why there has to be a lagging strand.. Can't both sides be replicated in a 5´ > 3´ direction at the same time? Aren't they symmetric? I mean, one side being replicated 3´ < 5´ and the other 5´ > 3´.

Also, how are the "leading" and "lagging" strands determined? Is there a kind of promoter in one side and the other side becomes the lagging strand? If this is the case, can't there be nearby promoters in opposite strands of the DNA? I'm having trouble figuring out what brakes the symmetry in the DNA...

If there was no lagging strand, then you would have to wait for helicase to uncoil the entire DNA molecule first.
1. This would result in replication taking forever since you'd have to wait for helicase first.
2. Not starting replication would leave DNA in single-stranded form, which leaves it open to coiling on itself, forming secondary structures, and being damaged.

The leading and lagging strand is simply determined by whether it goes in the direction of the fork or away from it. As far as I know, the cellular machinery does not recognize the leading and lagging strands as any different from one another. If there's an open strand, replication begins.