Instill Energy is a card that was originally printed back in Alpha and arguably it still hasn’t received a full errata. Nevertheless it is still a highly useful card and is the card of the week.
An enchantment aura for a single green mana that enchants a creature. Not necessarily a create you control. It allows the enchanted creature to attack as though it has hate. Additionally it allows the creature to untap one additional time during your turn, the full errata can be found here.
A large number of creature’s activated abilities requites the creature to tap as part of the cost. An untapper such as Instill Energy allows you to activate that ability a second time each turn. This allows you to either generate a large amount of value or Alter the board state to propel you ahead in the game. Enchanting Selvala, Priest of Titiania or even a Birds of Paradise will give you access to a lot of mana. Whereas Yisan or Vannifar would allow you to tutor for two creatures to the battlefield.
Additionally it allows the creature to attack as though it had haste. This is the more underrated aspect of Instill Energy but is very useful. If you are playing a beat-down deck it can enable your big creature to attack immediately. It can also act as pseudo vigilance, as you can attack with the creature and then untap it afterwards.
It can also be used in a political tool as it doesn’t need to enchant a creature you control. The only downsides are that is only provides attacking and not tapping haste, and you must untap the creature on your turn.