Essence you gain essence from battles bosses. Billy Shears talent also gives essence. Essence is used to summon spirits in Everclicker. These spirits are vital to your EverClicker play, they give boosts for pretty much anything from gold, damage/dps, items, idle play, renewal and greed. Each of these sets of spirits help with each area in different ways. Be choosy on whom you summon
Damage spirits, of course we know these help with damage. Read each one closely you have some that help with idle play, click damage and weapon enhancement. Gold and Items are easy these help you gain, items for spell crafting hero quests and weapon enchantments. Gold for leveling heroes. This also helps with new game sets. Idle play is when you're not clicking and the game is in idle mode. These allow you to gain gold and bonuses while in idle mode.
Now let's move on to Greed and Renewal. Greed well any of these I can get, I'll summon Greed spirits give allow more essence to be gained, Heroes cost less to awaken and hero powers can last longer. Renewal basically is how you start each new sets. Neireen is one of my favorite, allows your heroes less time to renew after used.