When I should hide a toolbar in an Android App?

I'm designing an app navigation flow. It has a login screen from which we can go to a "Forgot password" screen. Once we logged in we go to the "Main screen" there I've a toolbar with a gear item that goes to the "Profile screen". I was wondering, when should I show a toolbar? should I show a toolbar even when there's no items?. For example in the "Profile screen" I've a toolbar but the only thing that It displays is the back arrow and the screen title. Isn't a waste showing a toolbar there? Is there some reference about this in the Material Design documentation?.

I've been searching through the documentation but I couldn't find any answer that explains when we should show a toolbar and when not.

UPDATE

In the main screen I've two tab children, one of the tabs displays a feed list, the other tab displays a list of items (work tool items, crowbars, shovels, helmets, ropes, etc.) to buy. In the main screen toolbar you've a gear icon in the top right corner, there you can go to the profile screen. In the profile screen we've some information about the user, and that's all, just name, username, and a picture.

UPDATE

Also keep in mind that I'm talking about Android Material Design.