There is a lot of jumping around the control panel when setting up an EE site (channels, custom fields, categories, etc.) and this can often slow down your development process. The idea behind Developer is to provide you with a single tab that allows you to access all of these sections much quicker than usual.
Developer automatically adds a tab to the end of the main navigation of the control panel for super admins. It allows you to quickly access your templates, snippets, global variables, custom fields, channels, categories, member groups, upload destinations, modules, extensions, accessories, fieldtypes, plugins, admin links, tools and general configuration.
Reviews taken from devot-ee