On his Medium.com site Benoit Schneider (CTO of Outsourcify) has a post sharing some of the reasons they made the choice to use Symfony for their web development needs.

At Outsourcify we work on projects of varying sizes, from small sites with a few pages to complex business applications. Depending on the case, we recommend different technical solutions (we do a lot of Javascript SPA and Wordpress also), but for the most complex cases, when we have to chose a technology to build large web applications that require several weeks or months of work for several web developers, Symfony is our framework of choice.

He starts with a bit of background on his own experiences with Symfony and why he chose it for his own needs early on. He then shares his thoughts around "why Symfony?" when there are many other major frameworks to choose from. He wraps up the post with a summary of some technical reasons to use the framework including the innovation it brings to the table and the robust ecosystem around it (and related products/tools).