Canvas Quizzes does not currently have the option to grade student open ended (essay question) answers off line. Even the ability to download these essay question answers as a .txt or .docx or even .pdf file would allow for online grading. Ideally, these text downloads would include the student name and student number and, for anonymous grading, just the student number would be helpful.
I do know that student answers can be downloaded as a CSV through Quiz Statistics but this medium does not lend itself to grading essays.
Aside from grading, downloading student quiz responses also allows an instructor to compare student answers or more easily rate them relative to each other.