Cast Iron

You can cast it in desired shape, that reduces machining time and efort.also the grephite contant in it work as lubricant during machining and during its operation.

Its not always the case, sometimes mild steel, other times aluminum, not to mention others.

Why is Cast Iron used? You need to first look at the particular application and look at all the factors, some are;

1- Weight considerations y/n?

2- Number of parts required

3- Budget available to construct each case

4- Geographic location and available resources

Man it just goes on, and on, and on... But your question was really quite open ended, sort of like why do we buy sliced bread?

"... You need to first look at the particular application and look at all the factors ..."

All good points.

Cast iron isn't a "new & exciting" material, so some people are quick to dismiss it. Cast iron has some fantastic engineering properties.

Cast iron is the cheapest construction material suitable to absorbe lot of machine vibrations. Weight of casting gives required stability of the machine and dampening effect for the the forced vibrations. Natural frequency band of casting is nerrow and far from the normal running rpms of such machines so the problem of resonance is also not encountered.