The pressure pattern over the
wing goes from high at the leading edge to low at the point of maximum camber
and then back to high again at the trailing edge. So, from the point of max
camber backwards the airflow is moving from a low pressure to a high pressure
region - called an adverse pressure gradient. The air would not do this unless
it was being pushed by some other factor and this factor is the kinetic energy
of the moving air. The boundary layer has less KE than the freestream
air and gradually slows down. When it stops or reverses it breaks away from the
surface, or separates.
As the laminar type of boundary
layer has less KE than the turbulent type it will slow down quicker and break
away earlier. So the laminar BL has less KE and breaks away earlier. The
laminar layer, however, causes less drag.
To try for the highest possible
lift at high angles of attack you need to keep the airflow attached as long as
possible and this means having a high KE boundary layer. Various high lift
devices are there simply to re-energise a BL that is
slowing down. Never mind the drag, it is lift we want
at the stall.