Why would you need to consider scouring yarn for tapestry weaving? And what does scouring mean anyway?

Scouring is simply the textile word for washing. Some (very few) yarns come with machine oil in them from the carding process and that needs to be removed before using the yarn for tapestry weaving. This usually only happens with coned yarns because the yarn goes right from the carder to the spinning and onto cones without any other processing. The oil is added in the picking process so that the fibers go smoothly through the big carding machines.