confused with energy block"test extracting <> energy at...... return amount extracted"

according to the Klemon's response " Test extracting will try to extract energy but not actually do it, just report how much it was actually able to extract.

If you directly extract say 100 and block only has 50, you can not know this without testing the extraction first." 

So if I want to know how much energy I could extract from a block, why dont I just use the block " get the energy of block at x y z" directly?