The order of calling app.use(), app.get () in express

  node.js, question
app.get('/',photos.list);
 app.use('/', routes);

The original intention was to replace routes with photo.list, but why put get on top of use?
What is the relationship between them?

The photo.js file is

var path = require('path');
 var photos = [];
 photos.push({
 name:'node.js logo',
 path:'/images/tu01.jpg'
 });
 
 exports.list = function(req, res){
 res.render('photos', {
 title: 'Photos',
 photos: photos
 });
 };

Use is to load the middleware before get. Get post is a route, placed at the back.
If your problem is that you can’t execute the get after putting use in front, then check if the inside of routes function existsnext()
In addition, I don’t understand your original intention a little bit. The two methods are all function except the first optional string of parameters (specifying the route of execution middleware). Why are there variables like photos.list that look like arrays? . .