Jinja renders Python strings without the quotes. The user tries var x = {{ 'value' }} and that renders as var x = value. Whatever value is,it isn’t valid when JavaScript runs it,so the user sees a weird value or gets an error,rather than the string they expected.

Using {{ value|tojson }} produces a JSON string,and since the outer quotes are removed when Jinja renders it,it becomes valid JavaScript with the corect type.