I do so to either soften brittle feathers, or, help slow down extra fast shuttles.
The process is more useful for players in countries which have lower humidity levels eg North America, New Zealand, Australia etc.
Best advise is to tryout a new tube and then examine the shuttle fatalities after the session, before deciding on steaming.
The opposite of steaming is to dry the shuttles. Had to do so on a batch which seem to have a softer cork head eg more shuttles with lob-sided rib-cage structure at the end of a session, with the same regulars.
One internet source which delves into moisturising shuttlecocks is http://www.shuttlecock.com/Resources/Shuttlecock/hydrate.php
I am in the process of purchasing a purpose built humidifier for shuttlecocks. Will post once I obtain and had a chance to use the device!