In general, in science you are supposed to check and verify every step of your research. If one just uses a piece of software as a black box and proceeds to apply it to an unknown case, there is no guarantee that the result is trustworthy. One should have test cases where there exists an expected correct answer and test your tools.
In this regard, open software, for example community codes have been tested by the community and personally I think they are more trustworthy in comparison to scripts written solo.
Nowadays peer review has been rendered effectively worthless because nobody can check your codes to see if a simulation or analysis was performed correctly. Publishing your code or even better using community codes ensures a bit more, that the results are trustworthy.