Pretty much as Daniel put it.
Most often, it's used when one quotes a statement that contains a typo to highlight that this is not a mistake in quoting (otherwise the reader would have been left guessing who made the typo or used a weird word - author of the original text or author of the quote).
It may be important in formal texts (e.g. I very often see this in sophisticated legal documents) or when someone specifically cares of the linguistics and style of the text and wants to transmit the 'flavour' of the original text to the reader of the quote.
M
[DOUBLEPOST=1482361030][/DOUBLEPOST]I mean, you never know if someone copy-pasted the quoted text or typed it manually, which is when errors could have been made while typing the quote (and it does happen).