The reason is that not all mats are accurate in their measurments. I think it'd depend on what you were cutting, some things like HST takes very accurate measurments and others not so much. If I'm doing a large square for emb, etc then I'll use my lines on my mat. I always use them to make sure my fabric is straight before I use a ruler to cut it. It's more important to use the same rulers thru out a whole quilt, etc because not all rulers are the same and you can have problems using different ones in a project.