It is one of the fundamental commandments of gun safety, in fact. The basic reasoning is there are only two reasons for putting your finger on the trigger of any firearm, regardless of condition:
1. To knowingly and willfully discharge the firearm at an intended target.
2. To test the functionality of the firearm's action, after first clearing the firearm to ensure it is not loaded.
Many firearms require to pull the trigger when you want to uncock them. That is one of the reasons I selected a P226 which has an extra lever for it.
Basically you are right. If there is no purpose that requires a finger on the trigger, donīt touch it.