Why is there an emacs keybinding for character transpose? There is a special keybinding for transposing two characters in a text field: ctrl-t. Is this functionality really that useful to deserve a keybinding? I want to know what I am missing by not using it.

Side question: Can I remap ctrl-t to something else using for example terminal?
There’s about a gazillion and 3 Emacs keybindings. You’re not missing out on anything if you don’t use it. Why is it there?  It came with the technologies that Apple acquired that made the basis of OS X. 

What you should remember is that Emacs was born in a world that didn’t have advanced editors like we have and take for granted now. You’ve already developed your “workflow” on how to deal with typos in this modern environment, but way back when, having text transposing functions was a lot easier than taking the editor out of insert mode, putting it into overwrite mode, retyping the characters, then returning to overwrite (yeah, that many steps). 

As for remapping keys, you can, of course, remap keys. There are many tutorials and software recommendations on this site dealing with that very topic.