"outta" is a way of saying "out of" and therefore also a way to spell it.
There's no need to use this 'word' unless you are trying to behave like a certain person or act a certain way, but it is useful to be able to understand in case you do see/hear it.
There's nothing wrong with using proper English, but there's also nothing wrong with using whatever words (or non-words) you want. Just make sure you understand how other people might view your use of language.
Similarly, there's no reason to use:
IMO (instead of "in my opinion")
LOL (instead of "haha")
g2g, ttyl (instead of "got to go, talk to you later")
ur (instead of "your" or "you're")
but they're not all viewed in the same way.