Exercise without progress tracking can lead to loss of motivation and a slower rate of improvement. It is important to know where you stand on a fitness level so you know how far you have come and can take steps to keep improving. Many assessments people give themselves are subjective and simply based on how much they can lift or how far they can run. While these may be somewhat useful indicators, a more specific set of tests is needed.
Check out this Men’s Health article on how to assess your current physical state.