A financial model is always in motion (and should be designed as such), assumptions keeps changing all the time, the coding is optimized, new scenarios are added and you quickly end up with tens or hundreds of versions of your model just after a few weeks.
Then your boss asks you what the latest numbers look like. So you print out a few tables and graphs for him and suddenly he takes from his drawer the same tables and charts that you had printed out a few weeks earlier, but of course the numbers have changed. The NPV which used to be $452m is now $312m. "What happened?!" he then should logically ask you, and if at this point you had forgotten to add a changelog in your model, the next few minutes might not be the best of your life as you would likely struggle to explain precisely what changed since the previous version...
This is why a changelog is absolutely needed in every single model, with it you can explain in detail any change in the results of your model. But you have to be very careful as you need to record any change that you make in your model which results in a change in your results. To help you do so I have just published a tutorial that will show you how to create a semi-automated changelog that will save you precious seconds every time you make a change to your model.
You can find this tutorial here.