The 'Delete' ( on Mac / Backspace on Windows ) key fixes those perfectly. You type something incorrect, tap the delete key and fix it.
The key the OP is referring to is the windows Delete key. Which is somehow used to delete mistakes before they happen, by deleting the word before you type it. I made one typing mistake in this paragraph. Delete key fixed it. Backspace key would have been no help, because I had not typed the error yet.
I'm confused to why that key is SO important to the OP, especially on a touch interface. Just tap behind your error and delete it.
I think you have the Delete and Backspace keys mixed up, but never mind.
The Forward Delete key is useful when editing documents. For instance, I typed up a paragraph yesterday, then decided I wanted to replace all instances of "stop" with "lock." I know I could search and replace, but there was only three instances, I figured I could do it fast enough manually. So I deleted the last instance of "stop," typed "lock," then arrowed up to the previous line, where the "stop" was easier reachable with the Forward Delete key. Now, that was on a desktop system. 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.
This is just a single example, but the fact is, when typing on a desktop system, I use Forward Delete as often as Backspace. People who don't see the usefulness of Forward Delete either must never edit documents, or just have never used a system with Forward Delete, so don't realize how much cursor movements (or in case of touch screen, jumping back and forth by taipping around) a Forward Delete key can save.