1. It gives you encouragement and motivation to keep up the good work. When you see how much effort you have already put into something, you won't feel like giving up.
2. It can give you insight as to when you are accomplishing things and when you may not be. Armed with this insight, you can improve how you get things done.
In Tagaabo, have you noticed that when you check off a To Do item, an item tagged with "done" is automatically created?
By looking at those "done" items in calendar view, you can get a sense how and when you have been getting things accomplished.
By combining some other tag and the "done" tags, you can also see how you have been doing in a particular area. Here is how I have been getting things done on the development of Tagaabo.