Wood changes when it dries. Some bits stay the same, some bits shrink a bit, and some bits shrink a lot. As well as distorting the wood, this causes stresses which can pull the wood apart, resulting in cracks.
The worst time for this to happen is after you've made something - your beautiful bowl, or piece of furniture, or trinket box develops an unsightly crack somewhere obvious. To prevent this, the final working of a piece of wood should be when it is completely dry.
Unfortunately, wood dries very slowly, and often unevenly - it dries more from the ends of the grain than it does across the grain. To speed up the drying, especially when in a changeable and unpredictable climate, it helps to have some form of heated dry box to store the wood in, often called a kiln. Whilst the word "kiln" evokes pictures of super-hot clay firing structures, it can be as simple as a lightbulb in a cupboard heating it up by a few Kelvin.
Inspired by a setup I saw on Jim Sprague's YouTube channel (see below), I decided to make a small unused freezer into my own little kiln.