Vega can let us do things that Vega-Lite can't - additional signal, hover, highlighting, tooltip actions, and so on. We can take basic VL charts and convert them to functional Vega spec as a platform for adding additional items to them, using (vegaLite.compile(SPEC, {}).spec).
Here's chartSpec from Data by ZIP:
Look at the console to see the same chart spec formatted as Vega. Using the same embed code, vegaEmbed('#vis2', spec2), we embed the chart, now a Vega chart, into the page.