An extension framework on top of Backbone.
It simplifies your Backbone application code with robust views and architecture solutions.
Marionette is not an all or nothing thing. You can add it as you need on top of a large app regardless of what it is built with.
And you can build on top of it.
But Backbone does not help you so much on the view layer.
Implementing your own render solution
render: function(){
this.$el.html(`....`)
}