It is common to find this kind of detailing and patterning on engine blocks and related parts. Older engine blocks display much more flatter and simple geometry and this kind of mold work must come with a considerable production cost. What is it's purpose?
This pattern is to provide sufficient strength while minimising the mass of the block.
These "webs" are designed to prevent any vibration, if the block wall was made thin and the full length and width it would buckle or fail under the loads / stresses applied.
This design allows the wall to be thin in-between the webs so reducing the mass and helping to reduce the overall mass of the vehicle and help fuel consumption.
Also, you can see how the bolt holes or fixing points are re-inforced with extra material.
You mention the "extra work" necessary, but also consider the amount of material saved per block multiplied by the number of blocks produced...
As mentioned in other answers these patterns are structural stiffeners to give strength to a lighter engine block.
But they also have the task of distributing stresses due to internal vibrations of moving parts and concentrated stress of attached accessories such as exhaust manifold.
That is a big help in increasing longevity of the entire engine assembly, reducing fatigue cracks in the long run, and bringing the maintenance cost down.
These fins are also designed to make the engine run quieter and smoother.
There are two closely related reasons.
The first is that in terms of pure geometry webs tend to be more efficient in terms of strength and stiffness to weight ratios compared to a solid block of similar dimensions. You also see this a lot in fabricated and pressed structures.
The second is that in castings it is generally desirable to keep section thickness as consistent as possible and within a certain range to reduce the potential for casting defects related to shrinkage. It is fairly rare to see quite such intricate webbing in an engine block as it somewhat complicates mould design and in many cases an over engineered approach is preferred in terms of stiffness and vibration damping and where weight is critical it tends to be easier to change materials rather than produce highly optimised castings.
I have never seen that, very interesting. I expect the webs are stiffeners as mentioned. But I expect thin flat sheet of metal will cover the surface shown to enclose a water jacket. There are not enough fins for air cooling. so there must be a water jacket.Is it iron or aluminum ? It looks like aluminum but I don't see steel/iron cylinder liners.