I think the second answer is particularly helpful:
You will be using the same express() instance in app multiple times (for listening and HTTP methods (https://expressjs.com/en/starter/basic-routing.html), as it saves (stores) a single instance of the function express into a variable.
If you do express().use() instead along with other methods on express(), you will be creating multiple instances/applications.
In terms of naming it app, I guess you can name it something else, but it is convention, as you are building a web app (Express is a web application framework on node.js).