Macbook keyboard:I think you have the Delete and Backspace keys mixed up, but never mind.

Type something incorrectly... press the delete key to move the cursor back one space, deleting the error.
Windows keyboard:

Type something incorrectly... press the backspace key to move the cursor back one space, deleting the error.
The OP mentioned the mobile keyboard specifically.
Hold your finger for one more second and you can delete the entire word with a single click. Much faster than tapping backspace ( on window's keyboards ) to take out half of onomatopoeia then delete to take out the other half. That's a lot of clicks.On a touch system like the iPad, having Forward Delete means I don't have to be so precise when I'm placing my cursor. I can tap in the middle of the word I want to delete and forward and backward delete until it's all gone.
I understand, you have been taught to use both, I have been taught to use one. I don't miss a windows Delete key, where you would. But when designing a non-windows device and you have limited space for a digital keyboard, why would you add one when you don't even add one on your normal keyboards as the OP suggested.This is just a single example, but the fact is, when typing on a desktop system, I use Forward Delete as often as Backspace.
(Apple does have an usb keyboard with a number pad on the right that does have a 'Forward' Delete'.)