I am Abomination
The Orzhov welcome you. Please leave your belongings with the Obzedat. They are not yours anymore.
Hi, I'm a european crested tit, and a very small punk rocker!
So essentially you can never discard a card unless it was used for some purpose.
I'll add that some cards allow you to discard other cards, in order to gain a benefit, without the card being played for its own effect.
Such effects would be Essence Theft, for instance, which lets you discard 1 card in hand to heal a player 1 life. A gem lets you discard 1 card so an ally can draw 1 card.
These effects don't happen at the end of your turn but they let you play around unused spells and relics anyway and help you draw a new hand when necessary.