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.
List of all KnownIcons
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.
The Cache node allows you to explore in more detail how much is store in the various caches in Dynamicweb.
You can read more about the Deployment tool in this article.