If I take your question literally, then I cannot give you an answer. If my society deliberately erases the past, I probably won't realize it; therefore, I won't know of any advantages or disadvantages.
But seriously, I could see some benefit to forgetting the past. Think of the pain and anguish that would be gone if we blocked out all knowledge of the World Trade Tower or the Holocaust. We wouldn't need to worry about the mistakes we've made, because no one will remember them. Every team could have a winning season every year.
However, I believe that there are disadvantages to forgetting the past. It is not enough to say "those who forget the past are doomed to repeat it." It is true that we could be easily duped into believing anything if we didn't know better. But by forgetting the past, we forget what made us who and what we are. We grow by learning from our successes and our failures, not by forgetting them. By memorializing 9/11, we show that we are more than just flesh and bone; we have a spirit, a conscience.
See the articles I have linked below.