A lot of people want to know the length of the yarn as much as the weight. With length you can be more accurate in how much yarn you will need to finish a project or determine whether you have enough yard to make that extra square or finish your weaving project. It sounds like a chore but it is not hard if the wool is skeined. If it is a ball then is is not so easy. If you are making your own yarn then you need the weight and length so you can work out how much you will need to spin to create your project. Many patterns now come with both weight and yardage for yarn . You can buy electronic measuring devices to calculate yardage as you make the project.That will help if you want to duplicate the project with balls of yarn. There is some help here and in the video as to how to master calculating yardage.