You can fetch silver bullets against specific strategies
If Gogo copies a legendary creature, Gogo will also be legendary, but because its name remains Gogo, Mysterious Mime, the "legend rule" won't cause either of them to be put into the graveyard
That ability means that Jafar can always be on the offensive, doing damage and earning lore as he cycles out the same set of Illusion-based characters
Other times, as with Daru Stinger above, it meant giving the creature an ability that cared how many +1/+1 counters were on the creature
When it enters, you can have it copy any creature from any graveyardfriend or foewhile keeping its 4/4 spider human hero identity
If you've got even a semi-wide board, this could be a seven-card swing - especially if you had less than four cards in hand to begin with