ExpressJS : Why should I seperate my routes and the functions handling them?

I've recently started a Nodejs project and my roads look like this :

router.get('/groups/', function(req, res) {
  GroupManager.getAllGroup(function(groups) {
    return res.json(groups);
  });
});

I saw few example of people separating their routes and the function handling them, leading to something like :

GroupController.js

exports.module = {
    getAll : function(fn) {
        GroupManager.getAllGroup(function(groups) {
            fn(groups);
        }
    }
}

Routes.js

router.get('/groups/', function(req, res) {
  GroupController.getAll(function(groups) {
      return res.json(groups);
  });
});

The project I'm starting is growing pretty fast so it's very important for me to have a clean codebase, but I struggle to find the benefit of this way of splitting the code.

Also, I'm struggling trying to find some documentation about code organisation under expressJS. I found many blog post discussing it, but nothing that made authority, is there some famous open-source expressJs project I could use as inspiration ?