People spend most of their time searching for things that they referred to earlier. It's quite often that when we need things the most we don't find them in right place. We as human beings tend to perform multiple tasks, and during this process, we forget most things.
We are so busy in our life that we always need some sort of reminder which tells us what to do and where to find things. It's really useful to bookmark thing while reading something or while browsing the net because there is plenty of things available around us keeping track of all of them at one go would be really difficult.
A bookmark is like a reminder that helps us to resume where we left things and it makes our life easy.