tl;dr: This procedure checks the oil "reserve" remaining in the pan after all moving parts are lubricated.
This procedure is purely vehicle dependent. For example, on my car, I'm supposed to check the oil when the engine is cold.
In asking you to check when the engine is hot, the oil will not be pooled in the oil pan. Instead, it will have been fully circulated to all the moving parts that badly need that oil to function. As a result, you'll be checking the oil reserve that remains in the pan.
Note, this oil isn't extra. Among its many functions, oil is also carrying some of the heat load of the engine. All those hot moving parts get the oil in contact up to painful temperatures. An "excess" allows the oil to absorb more heat without burning completely away. The oil in my car leaves the block, passes through an oil cooler and then to the oil pan before circulating back into the fray.
Note: I can also check the oil in my car when hot by using different marks on the dipstick. I prefer the cold procedure so I can confirm total oil.