Why is it worth using Symfony?

Strategic point point of view:

1. On the market since 22 Oct 2005 and is now a framework and a set of reusable components which are used by projects such as Drupal, Magento, Sylius, Joomla and frameworks such as Laravel, Lumen, OroCRM, Pimcore and more.
2. Among Symfony creators there are people who created and are managing a dependency manager widely adopted in PHP community: Composer  - Nils Adermann and Jordi Boggiano and who contribute to Behat - like Jakub Zalas.
3. It has a history of keeping backward compatibility and applying Semantic Versioning. So you may be much more confident that when you update from version Y.Y.Y to Y.Y.Z the changes in framework won't influence the code you've already written.
4. +2000 contributors, +300,000 developers
5. Symfony is distributed under Open Source MIT license which is free to use in commercial or/and proprietary application.
6. Developed by SensioLabs, an 18-year-old  web agency.
7. It has a foreseeable, time-based schedule of releases - Symfony is released every 6 months in May and in November and is maintained for 8 months. LTS is released every 2 years and is supported for 3 years. 

Technical point of view:

1. Symfony is a framework that's goal is to speed up creation and maintenance and replace repetitive tasks. (Doing that thanks to bundles, adoption of Dependency Injection, SOLID, low coupling and high cohesion).  
2. Symfony was heavily inspired by the Spring Framework (so it is architectured similarily to Spring). When googling for answer you might come across the same concepts from coming up on pages for Symfony and Spring.
3. Easily works with ORM such as: Doctrine to which commonly used is Doctrine Migration Bundle 
4. Web debug toolbar - feature rich toolbar for development. 

• so you have info about latest request:
  • http status
  • Controller
  • Total time of generating site
  • Memory usage and memory limit
  • Render time and number of template/block/macro calls
  • Database queries, invalid entity definition warning
• It can be extendable. Example? This Doctrine profiler bundle, written by a team in Wrocław @Pixers https://packagist.org/packages/pixers/doctrine-profiler-bundle

5. Symfony creators also created Twig - a templating engine.