All cookie doughs do not require chilling so you can skip it sometimes. But when your recipe calls for it or your cookie dough is soft and greasy, chilling is a must. Or you will end in flat crispy cookies. They won’t be as soft as you would want.
Another reason to suggest chilling is the cold centre of chilled cookies cooks slowly compared to outside. Giving you chewy outside and super soft inside.
This step is also important because it gives flour and other ingredients time to merge together, and hence provide heightened subtle flavor.
Chill dough overnight for best result. If you are in hurry, scoop cookie dough baking sheet and keep in fridge for 30 minutes. They'll chill quickly.
Lastly, if you chilled dough overnight and it is very cold, allow it to rest on counter for about 10 minutes so it is not very cold when it reaches the oven. If you don't the bottom of will be golden and center will be still undercooked.