
First of all, if you do this with a piece of meat or fish that you put in the oven right away, it will cook, of course, but not as well as if it were at room temperature.
Once in the frying pan or saucepan, the outside of the piece heats up very quickly, while the inside takes much longer to come to temperature.
To compensate, we prolong cooking, which dries out the surface and sometimes results in an overcooked or undercooked center.
The thermal shock of a very cold piece coming into contact with a very hot pan can also stiffen muscle fibers, especially on fast-cooking meats (steak, escalope, poultry) and harden them unnecessarily.

It's also a (small) waste of energy for nothing.
Imagine, for example, that you want to boil 1 liter of milk to make a rice pudding: your milk, taken out of the fridge at 4°C, will take longer to heat than if it were at room temperature.

A very simple solution is possible: just take the food out of the fridge 1 hour in advance and leave it at room temperature, while it slowly comes up to temperature.
For meat and fish, slowly returning to room temperature means that heat penetrates more evenly and the core reaches the right temperature more easily.
What's more, by doing this, you'll also save energy, which is good for your wallet and our planet.