Varnish is a web application accelerator or HTTP reverse proxy. Simplified, this means that Varnish caches HTML pages or other HTTP responses and can deliver them to quantity at very high speed (sub 50ms HTTP responses) and in big numbers (several thousand requests per second for one Varnish instance).
Our primary use case for Varnish is Magento 1 and 2 where Varnish can help a lot with performance. Note that it is not trivial to setup and maintain Varnish in a project properly. If Varnish is not suitable for a project, alternatives can be CDNs also caching these requets or the built-in Full Page Cache (FPC).
TYPO3, Laravel, Symfony and other applications normally achieve very quick response times so we don't need to use Varnish here normally.