Becasue if it did not drain it would not be Ebb&flow, it would be DWC (Deep Water Culture), another common technique for growing in hydroponic culture.

The main advantage of E&F is that you do not need to try to get enough oxygen disolved into the nutrient solution to support the plant becasue the roots 'breathe' when the flow cycle exposes them to air.

I hope this answers your question.

Ahh, I see. Thanks. One more question though. When doing hydroponics outside. What do you do when it rains. Its possible to overflow the nut container.

Naaaah, don't fuzz,
It'll have to rain a lot for that to happen.
Check pH and EC after a heavy rain, then adjust as required with more nuts and/or pH-up/down.