If there is no written record of the past, then any given population is entirely dependent on their own memories. Memories are fickle and fade. They tend to fade more and more as generations change as well. If people can't accurately remember exactly what happened in the past, The Party can make up whatever they want about the past and sell it as truth. There is no way to disprove it, because there is no written record of it. It's a form of control over the population of citizens. The Party can completely turn around on who the enemy is, because there is no written record that they were ever in conflict with country "A" or "B." Essentially, by wiping out records of the past, The Party can make present day policy whatever is most convenient at the time. The actual quote from the book is as follows:
"Who controls the past controls the future; who controls the present controls the past."