Major mistakes are mistakes that matter. Where what you say (wrongly) can really lead to a misunderstanding (worst case), violates the basic structural regularity of the language, or is simply not the correct form to use or right thing to say because it has a different meaning from the one intended.

Analysing and discussing these mistakes can help understand the semantics of grammatical forms, especially verb structures. In the case of vocabulary, it can help improve your active use of words and phrases (and I find talking about words and their meanings enjoyable, but that might just be me).