By setting goals for yourself you are able to measure your progress because you always have a fixed endpoint or benchmark to compare with.
Take this scenario for example:
David makes a goal to write a book with a minimum of 300 pages. He starts writing everyday and works really hard but along the way he loses track of how many more pages he has written and how many more he needs to write. So rather than panicking David simply counts the number of pages he has already written and he instantly determines his progress and knows how much further he needs to go.