Before revision:  But classifying networks into tree-based and non-tree-based ones is usually not sufficient for biological purposes, because sometimes tree-basedness is missed merely due to incomplete sampling. Therefore, measures are needed to quantify how close a non-tree-based network is to being tree-based .
After revision:  But classifying networks into tree-based and non-tree-based phylogenetic network, however, might appear to be very close to being tree-based .