I think the division by 2 is simply to calculate the 'average' pressure on the side of the tank. But, if you ask me, that's not very useful if you are going to be doing some kind of calculation on the tank...you'd better be working with the maximum pressure...which happens at the bottom.
The pressure on the side of the tank is exactly the same pressure inside the water at the same depth...fluids exert the same pressure in all directions.
Does that help?

Yeah,That's obvious,I know that points on the same horizontal plane have the same pressure but What if was just asked to calculate the pressure on the lateral side of the tank?Should i calculate the average pressure or the max.pressure as you mentioned?
also how to calculate the total pressure on the lateral side?