I think everything is recorded and then deleted and not only recorded when it's 'on'. Flows need the inputs and outputs for later reference and I think everything is kept to end in either making it visible or not after the flow is done.

It will help in less log lines, shorter loading times of execution records and such, but while the flow runs, it is collecting the data. 
This has always been my guess (never tested it though). You could reach out to NowSupport to find out for real.