A player may also left-click a mob whilst sneaking to create a spawner of the Vanilla type that doesn't expire and will spawn mobs repeatedly, in groups, when the player draws near.
If you are holding the Mob To Spawner item and you right-click on a spawner block a mob will appear and the spawner will remain. You can right-click repeatedly on the spawner and multiple mobs will appear. If you left-click on a spawner the mob from the spawner will appear and the spawner will disappear. This is a useful way to make mobs spawn from spawners without going into Survival Mode.