1. There's a problem with the account (and then inform them and maybe notify the mods myself). Sometimes people get shadowbanned without good reason (usually the result of automoderation, not an explicit human moderator action) or for something that happened years ago, but their history is pretty clean since.

2. To see if the person is just a spammer (as is the case here).

Though I only do this if the comment seems like something that oughtn't be dead (as this one does on a first glance). The comment is shallow, but not necessarily wrong or bad. It just adds little to the discussion since it boils down to "The book is an introductory text." which we also get from reading the submission title.