Truth be told: I didn't even realize that irons came with this little cup until after I moved in with my husband and I watched him refill the water section of his steam iron. All this time, I was awkwardly putting my iron under the kitchen faucet!
Aside from cutting time (and frustration) of out refilling an iron, this little cup has proven super useful for watering other not-easily-waterable things. We got this adorable bamboo plant as a present during our housewarming party and, while it adds some green to the house (which is needed during these nasty winter months), it's not exactly easy to water. But, with the refill cup that came with our iron, I can water it in seconds -- and without water getting everywhere.
This is also good for any small house plant. I use it for those green onions I have growing in old tea cups. Basically any place where a watering can would prove inefficient is perfect for this little cup.
Who would've thought that the item that I didn't even know existed could prove to be so useful?