Storage drops off unwanted seeds into a compactor of your choice. They both do useful things, but not necessarily in the most useful order. In general, a dupe on stprage priority will do 2 or 3 less trips to get the goods closer to a workspace than a supply dupe who is delivering to...
2) The downside is that now CO2 will start to collect at the bottom level. Eventually your dupes doing research will start panting because of high CO2. That's when you open the trap door for a few minutes and let that nasty CO2 drain down into the lower part of the map. ON...