One reason, and possibly the biggest reason that any modeler will tell you is simply cuz of your model and curves. Some curves don't need it if you make sure they don't or they're extremely high poly, but doing that is a pain in the ass and time consuming.
However, simply clicking / for everything isn't a good idea for you model. some areas need fine detailing and care to make sure you've used / and \ correctly. there's a huge difference between \ and /, i.e. the lighting, the model itself, curves reletive to it.
Simply, make sure when you do use / or \ that you use the correct one. Don't half ass useing triangulate.
as for closing stuff, outside of shadows, if u can't see the face, doing put it in there or ur just wasting faces.