Hello Nomand_King,

Here is short explanation:

In developer mode without assets you have no access to the original assets files. As you know Odoo framework is compiling all assets (js, scss, css files) into single one for the opimisation. This makes harder to debug Java Script and CSS files. 

In developer mode with assets, you will gain access to the original assets files that had been included into the modules, just like they were - before compiling. So you got access to the original files, that really helps to debug and helps do develope and test js code.

NOTE: You won't see anything special diference inside Odoo after switching into assets mode. But the major difference you will see under the hood, when for ex. you will fire the developer tool and inspect the page sources. 

Below the video showing the difference of accessing the assets in both developer modes. Video replaces thousands of words. 

Hope this helps,

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good luck!