Well during flowering you are feeding your plants a shitload of nutrients, right?
If you just chop it down and smoke it without flushing, all that stuff is going to be there. Ions, metals, salts, etc.
If you give it a week or two to flush, your plant is going to stop taking in new nutrients and have a chance to metabolize all of the food that it has stored up in its leaves, etc. You can actually see this process happening, because the fanleaves will start to fade in color. The cool part is, once the dark green color starts to fade, then purple/red colors can start to show a lot more.
You can definitely taste the difference between flushed hydro and non-flushed.
If you want to try and experiment, try heavy feeding and then NOT flushing. Then cut some fanleaves and put them in the microwave.
I accept no responsibility for anything that happens to your microwave.