Why should we used WDM System?

Pls look at following picture for Example,For a 4signals optical transmission system,100km span with regeneration,if without DWDM, there need 4x100km fiber, and 4 EDFA; if with DWDM,there only need one pair of 4channel DWDM Mux&Demux,1x100km fiber,and 1 EDFA.The WDM System can save cost effectively through optical transmission system.