What is the problem with using a generic list of stop words? 

The generic list of English stop words may not be appropriate if the set of documents are all related to a specific domain. For example, if the documents are all about finance, words like “money” and “economy” might occur very frequently and provide little differentiation between documents. If that is the case, it is a good preprocessing step to append these niche-specific words to the list of stopwords to prune from the vocabulary before performing vectorization.