That's a very basic example. Let's say your program also depends on ffmpeg to convert some images, psql to interact with a database and a few other non-library

That's a very basic example. Let's say your program also depends on ffmpeg to convert some images, psql to interact with a database and a few other non-library dependencies. With containers you can trivially ensure those are always present and with the correct versions. Plus containers do give you some security benefits when compared to running natively.