Why do we take photos?

I spent an hour this afternoon to sort and organize my photos. Equipped with a digital camera, it is so easy to accumulate a lot of pictures, without proper naming and time stamp. And since it is free, we usually take the advantage to take many alike photos, so that later we choose the best. However, since harddisk space is so darn cheap too, we usually end up keeping them all. So it is necessary to take time once in a while to discard out-of-focus or redundant photos and to organize the remaining.
While doing so, it came to my mind that what the advantages are really to have photos of as much moments in our life as are possible? What did they missing, let say a century ago, when people did not have any photos of their past?
I was looking at some of the photos I have taken or was given to. From special moments, happy moments, celebrations, farewells, friendly gatherings, first-time moments, with somebody I love, of somebody I love, .... and of course sad moments.
Some seemed so like yesterday's that I did double check their time stamps, I could not believe they were, for example, 2 years old.
If nothing, they remind us of the good times, that our past hasn't been a waste; in addition to giving a warning that time is passing by so fast. Always be aware that "What is important in life is the journey itself, not just what you expect at the end of the road".