Hi Ioannis, an authoring tool is simply a tool. It doesn't do anything that you couldn't do yourself using enough custom coding or other tools. The point of it is to make it easier, faster and provide templates so you don't have to re-create the entire wheel.
You don't need a hammer to put a nail into the wall, you could use a shoe or a number of other things, but the right tool for the job makes it go much faster.
It also becomes important when you are working on a team or for a client. If you build it using a standardized tool, your team mates can open it and understand what you did and work together easily. You can also then hand the completed asset files to a client and they can use the same authoring tool you did to easily make edits, without having to understand all of your custom coding.