Purpose of filterEl.addEventListener('keyup', function(e)

I’m learning the mapbox gl js examples and have trouble understanding a function.
The function can be found here https://www.mapbox.com/mapbox-gl-js/example/filter-features-within-map-view/ it’s in fact the “filterEl.addEventListener(‘keyup’, function(e)” in the example.

I already removed the complete function and noticed no difference in behaviour.

This is the code:

 filterEl.addEventListener('keyup', function(e) {
        var value = normalize(e.target.value);

        // Filter visible features that don't match the input value.
        var filtered = airports.filter(function(feature) {
            var name = normalize(feature.properties.name);
            var code = normalize(feature.properties.abbrev);
            return name.indexOf(value) > -1 || code.indexOf(value) > -1;
        });

        // Populate the sidebar with filtered results
        renderListings(filtered);

        // Set the filter to populate features into the layer.
        map.setFilter('airport', ['in', 'abbrev'].concat(filtered.map(function(feature) {
            return feature.properties.abbrev;
        })));
    });