In my understanding, this is a literary device (technique) - changing verb tense when describing a scene from the past or future, from the perspective of someone who is there experiencing it in the present.
It can be described in past tense also, but when writing a novel, we can change the current time to the past, and speak from the present tense. There is probably a term for this, but I don't know what it is.
Anyway, it has the effect of "transporting" the reader back into the time which it occurred. We're basically writing from the perspective of being in the past, but experiencing it as if it were present.
does that make sense?
就說，我們想像，那時的情況怎麼樣？假想我們在那個時候存在，為了形容眼前的情況 (sorry for my strange chinese, I don't know how to say this very well.. but i think you can understand what i mean? 怎麼講?)