I do believe that there is a limit to what people can remember. Since my brain is a physical object that stores all information, I think that it is likely that all memories I have will be lost forever after I die. Because of this, it is even more relevant to write down as much as I can about my thoughts while I have the chance.
I often look back at things I wrote from a long time ago and see how much I have changed since then. Most of the time I simply forgot that I even wrote them. This same thing applies to the videos I recorded or even when I see old movies that I have watched many times before. Why do I not remember all these kissing scenes in these movies? I think that I forget things that are not relevant to the way I want to live my life.
Much of what defines a person is their memory. If someone loses their memory, they may still be considered alive physically, but they will not be able to do all the same things they did before. A memory loss would put them back into the same situation they were in before they were born.
My writing is mostly to help me remember things I have learned, but it may also help others learn those same things faster than I did. Human knowledge has only increased because of those who took the time to write. Those who did not write are either forgotten or are remembered by the writings of others.