> but there is nothing in the UI to indicate that the back button will go there either, just a mental note by the user!
I'm not sure where you expect such an indicator to be, but that isn't the problem. If the user can't remember where they came from, why would they want to go back? The same 'problem' must also apply to the double-tap of iOS. (Which I had never heard of until now, but I only use iPhones occasionally.)
Given that you seem to have no issue with the concept of global 'return to home' and 'return to previous app', both of which are places previously visited, it's rather contradictory to take issue with a method of going back which is at worst equivalent to those features.
> If the user can't remember where they came from, why would they want to go back?
Because "back" is often conflated with "out"--the user might simply be done with the task and wants to get out. Unfortunately, the app simply shoots them to the previous app on the stack, which may or may not be where they want to go or remembered they came from.
Ex: Many apps go to the browser, but the user might not remember which app sent them to the browser. They might have thought it was Twitter, when it was in fact the home screen.
To enlighten you, the home double tap in iOS does not take you straight to the previous application. It brings up iOS's multitask tray that has the four previously viewed apps on it, in order of recent use. The first app on it is always the previous app. You can see which one it is. Android doesn't show this to you, nor do you even have the option of getting to the previous app unless you are at the bottom of the stack.
Edit: We're arguing about the merits of having a dedicated back button. If an advantage to the iOS task switching feature is found in yet another button on Android, that doesn't contribute support to the back button's existence.
>Ex: Many apps go to the browser, but the user might not remember which app sent them to the browser. They might have thought it was Twitter, when it was in fact the home screen.
This is exactly what I like about the back button. Frequently I'll finish an article I was reading in the browser but not remember immediately what brought me there. (This might happen if I had to interrupt reading it for a moment to pocket my phone and use two hands.) I'll have a sense I'm supposed to do something with it, but what? Did someone text it to me? Was it in a Gchat? Email?
"Well, that was a good article... why am I here, again?" I hit back, and find myself in an email with a link to the article. "Ah, yes, that's it! Joe emailed it to me." Now I can reply to Joe about it.
Maybe this wouldn't be as helpful to others, but I like it because I'm quite forgetful.
I'm not sure where you expect such an indicator to be, but that isn't the problem. If the user can't remember where they came from, why would they want to go back? The same 'problem' must also apply to the double-tap of iOS. (Which I had never heard of until now, but I only use iPhones occasionally.)
Given that you seem to have no issue with the concept of global 'return to home' and 'return to previous app', both of which are places previously visited, it's rather contradictory to take issue with a method of going back which is at worst equivalent to those features.