Earth is relatively shaped like a sphere, so
maps we have of it generally show on a flat
surface (two dimensions) what is actually
happening over a whole volume (three dimensions).
We don't have to put North at the top of a map,
but we need to choose directions in a map to show
a world of three dimensions in two.
Historically, other directions other than
North have been chosen as "up" by different
cultures. One benefit of having North as up is
because the magnetic North is near the North
Pole. But Earth's gravity doesn't pull
us in the direction of the North or South poles;
it pulls us towards Earth. If it weren't
then we might see people fly off into the sky near
Water flows downstream because of
gravity; it moves from higher elevations to
lower elevations, which explains why the Nile
flows from the highlands of East Africa to the
much lower Mediterranean sea.