Of course, no memory is completely pure because it is tinged with our own feelings and longings. However, it can help us to recall things that we do not want to repeat and bring to mind what we would like to happen again. However, it must not turn into a nostalgic view of what was never that way because that traps us in the past.
It does not need any explanation. Everyone rely on their memory for knowledge. If you are taught a way of doing something, the only way to remain acknowledged of that is through your memory. So without your memory, you'd forget what you were taught and hence it no longer becomes a known thing to you, i.E. You don't KNOW it anymore. However if it stays in your memory then whatever it is you are taught will forever be a known thing to you.