Why do you think it is important to evaluate last? why shouldn’t you just form an opinion before you start reading?

It is important to know exactly what your basing your opinion on. It kinda follows the "Dont judge a book by its cover" mentality. You will not know how a doughnut tastes, and you can not say you don't like it before you try it, you also can say what the flavor is, or what it means to you.
Hope this helps!~Karma