The Developer section contains tools and settings primarly relevant to platform developers - though some, like the Image Handler tool, are also useful for implementers.
Most of these tools are only available to Administrator-level accounts.
You can read more about the Image Handler tool in this article.
The List of all KnownIcons page contains a list of all the KnownIcons available when extending the Dynamicweb backend with custom functionality.
For instance, if you create custom Deployment configurations, you can set a custom icon for each of your configuration nodes - see more here.
The List of all Knownicons can be found under Settings > Developer > List of all KnownIcons.
You can read more about the Deployment tool in this article.