I know I didn’t have to tell you how good this record is. I know I knew.

In that case, you told the other person that the record was good, in the past. And in the present you tell him that (e.g. “I know you did not have to tell him that) you know you did not have to tell him that,” says the Rev. C. E. Schultz.

What is so great about this record?

In that case, you don’t tell the other person how good the record is, because you know that you don’t have to. Both “I know” and “I don’t have to” the present exist.

The difference between both sentences is that you told the other person how good the record is & the second sentence, why not?