如果已经定义了Express路由，为什么要创建Angular路由？

I'm new in MEAN stack area and I have some doubts related to Angular routes. Why should I recreate on the client side the routes already made in backend with express.js, what are the benefits? Is this the only way how Angular.js works? I saw some examples with Jade.js and it wasn’t necessary to recreate the routes on the client side, making the things simpler. I'm new in MEAN stack area and I have some doub