They are only removed if the card instructs to do so, or through an action card such as Break
When Ureni enters, it deals X damage divided as you choose among any number of target creatures and/or planeswalkers your opponents control, where X is the number of lands you control
Whether you need lands to play or cards to discard, Land Tax has you covered
While that might seem pretty cool at first, now that there are no rules around what cards can and cant be played (other than the ban list), you now suddenly have thousands and thousands of cards you have to somehow choose from in order to figure out what deck/combo you want to use against your opponent
A discard outlet that costs five mana when your curve ends at five will not greatly enable any graveyard synergies you might have in an appropriate time scale
Continuing the recent trend of rewarding players for playing multiple Prerelease events, you can also earn a velvet pouch by attending more than one