# What are development tests used for?

This is a theoretical question. I am newbie to artificial intelligence and machine learning, and the more I read the more I like this. So far, I have been reading about evaluation of language models (I am focused on ASR), but I still don't get the concept of development test. The clearest explanation I have come across is the following

"Sometimes we use a particular test set so often that we implicitly tune to its
characteristics. We then need a fresh test set that is truly unseen. In such cases,
we call the initial test set the development test set or, devset"


Nevetheless I have not found sense as for why an additional test has to be used. In other words, why aren't training and test sets enough?