I have read that too and we try to follow it. As I understand it, the reason behind it is that by just saying 'good girl' you are firstly not justifying why she has done the right thing, secondly you are building her self esteem to be all about your opinion and doesn't encourage her to self-recognise when she has done something she can be proud of and why. So instead of striving to make you happy (which is a type of dependency) she learns to recognise for herself when she has done something new or great. Also it is also supposed to make them more resilient. For example if you always compliment her drawings by saying 'that's great' then when a kid at school says 'thats terrible' she doesn't know who to believe. If you say 'I think the colours you have used make the picture seem very happy', she can be better at self evaluating when the other kid says something negative.
that's just the way I understand it but others may have different opinions.