1 Ever heard of do not put all eggs in one box ?
it relates nicely to this.. if fuel tank was flat and it drained all then you are lost .. this design is a second reminder to rider to fill the tank with fuel.
if there were just reserve and flat tank design and once your bike stops after hitting reserve then you are done..
it has saved me many many times .. still get 1-2 kms after laying it sideways and giving few jerks.
besides second reason is that this fuel tank canot have flat bottom as it relate to its frame. so it has to has some pockets on either side of its mounting to 1: keep it stable on frame.. and help distribute weight..
also it is acting as a shock absorber for the fuel in the tank . (the ridge dampans the layers )
also it is maximizes the amount of fuel stored in the tank ..
so all in all its good and necessary measure. i do not see a single drop of fuel being wasted due to it..
hope these clears your confusions.