As wood is a natural material designed by nature to conduct sap up a tree, even when well varnished or oiled it can take up moisture from the atmosphere and expand, then shrink again in dry conditions.
It can also warp.
These movements are extremely tiny in a well-seasoned wooden stock of good quality, but they can affect absolute pinpoint accuracy.
Synthetic stocks, which are plastics of various types, are not affected in this way, and their stability in all conditions of heat, cold and humidity ensures constant accuracy once the rifle has been properly zeroed to your chosen load.
An alternative, which is just about as stable and possibly more elegant than synthetic, is a laminated wooden stock.