What Docker has Solved

I am new to Docker Ecosystem , I would like to understand the World before Docker and how does Docker solved it.

Please give reference to blog or tutorial which explains JUST this thing very well.

This one isn’t too bad: https://blog.codeship.com/why-docker/

Of course, since it is a year old, it is out-of-date in at least one particular… Docker for WIndows is now allowing the same sort of things for Windows as it has for Linux all along. Not quite on the same level, yet, but improving.