Say you have a python/rails/nodejs/c++/whatever app. Sometimes getting all the dependencies on the system is cumbersome and hard to manage. This is true for both developers, and the people deploying these apps. I can't count how many times I've had a python app fail to build on a new box because of some C extension and forgetting to install a package on (centOS, ubuntu, debian, ect).

Docker lets me do all of this once for my app, with a Docker image, and now when I want to deploy it, all the system needs is Docker installed. This means on my laptop, on our staging server, on our production server, all they need is Docker, and I will have the exact same environment in each place, and to deploy the app, its exactly the same.

There's a ton of other bonuses like each container's dependencies and processes are isolated from each other. I also get a ton of the Docker features that allow containers to communicate with each other and setup service discovery between them (ex: your database container can now expose information to the app container using environmental variables).

Tons of other good reasons too though, you should check it out.