The short story is that all materials have internal stresses when made. Whether it’s lumber or steel or cast iron, they all have stress that need to be relieved before you can expect them to hold their shape. Some materials are worse than others. And anytime you cut or machine them, they can move in unexpected ways that can make your parts not fit together as required to make a working machine.

A “green” iron casting has a LOT of internal stresses created by the rather violent process of making the casting. Even way back in the day, they understood the problems that those green castings had. And if you want your lathe to be stable enough to hold those tight tolerances to build a train steam engine or bore an accurate cannon barrel you needed to get as much of that stress out of those lathe bed and head-stock castings as possible before you carefully machine and scrape the ways on your lathe into perfection so that the casting becomes as stable as possible.

The best method of relieving those stresses from your raw castings was to repeatedly heat and cool that casting. And the easiest and best way to achieve that was to literally store those castings outside for a few years to let them naturally heat and cool with the changing seasons. If you go on YouTube, you can find videos of British steam engine manufacturing from start to finish. And at some point you will see their outside yard filled with raw castings aging in the natural heat and cold of the changing seasons.

We still age cast iron to today to make it stable enough to use. But we now use accelerated methods of stress relieving metals that are much faster, but more costly.