Having recently watched the movie "Today's Special" (2009, Aasif Mandvi), I noticed a recurring theme that I've seen in a number of cooking-related films: the idea that a cook should "cook from the heart".
In the movie, the cook is repeatedly told that he over-thinks his cooking, that his work has no fire, that he needs to stop measuring, and that he should listen to his heart and gut. I don't think it's much of a spoiler to say that in the end, when he throws caution to the wind, his food turns out to be fantastic and the customers are happy and everything is right in the world.
The same basic theme pops up in "The Ramen Girl" (2008, Brittany Murphy).
Here the main character is told to cook from the heart, and to put her feelings into the ramen soup.
Don't get me wrong, I enjoyed watching both of these movies. They're both light-hearted romantic comedies that involve food - fun to watch, easy to forget afterwards. That being said, I think they reinforce a belief about cooking that is inaccurate and that can discourage novice cooks: good cooks never measure.
It may be that some good cooks don't need to measure out precise quantities of ingredients, but they are always measuring. They evaluate the smell, taste, color, texture, and consistency of a dish almost constantly while they work. They have a built-in knowledge of the size of a teaspoon of spice, a tablespoon of oil, or a cup of milk. They weren't born with this knowledge though. They gained it by making the same or similar recipes over and over, hundreds of times, until each bit of information was permanently ingrained.
Because they have internalized all that information, they know which ingredients to be precise over, and which ones allow lots of leeway. To an outside observer, it may look like the cook is just throwing ingredients into the pot haphazardly, but with each one there's a mental note of "That seems about right." That's where the "heart" and "soul" come into cooking.
If an inexperienced cook tried to work the same way, the results would likely be less than ideal. They may get lucky, but sooner or later they'll end up with an inedible mess. Therefore it's vitally important for new cooks (or even old cooks trying new dishes) to follow a recipe. This gives them a basis for comparison, which they can then change as their skills and experience allow.
Unsurprisingly, this also applies to cooks who try recipes from medieval sources. To make the challenge more difficult, such sources typically not only lack measurements for quantities or temperatures or times, but they can even lack the common basis of experience that a cook can use to measure these things. The cook must rely on their modern experiences and constantly question their assumptions. Sometimes it works and the dish turns out well. Sometimes not, and the cook has to dig in to figure out where it went wrong. In those cases it really helps if they've measured the ingredients.
Most importantly, if the cook has measured, they can write the recipe down and share it so that others can learn.