Well,

when you enable developer mode for the user Admin,

You will gain access for internal & technical properties,

Say for instance, Under Settings, by default you will be able to see only couple of menus such as Users, Groups, etc.

But when you enable Developer mode, you will get to access many key-technical-features which are mostly configurable only once and/or at the time of installation, such as Email, Parameters etc,

In User Form, you will get access to internal individual Group access to change the basic permission for an user,

Similarly in all Menus/Action, you will get access to see its View such as Form, Tree, Action etc, which also lets you edit for time being without undergoing the process of changing the code and/or restarting the Server, or to change xml-views and upgrading to see any changes,

further you can get all the technical properties/info for each and every fields present in the Views, which are much needed entity for any developers.

So if you are a developer, then these feature are beneficial to get database-level properties and other options which can be used to change the basic configuration, If not developer then it can be just viewed as initial or one-time configurable entity.

So why don't you enable and see for yourself to get a better insight of it.