Its essential part of essence basically.
When its on you get 300% exp boost, if you use extra boost (sayha storm) it gives 200/300% extra on top (based on version of storm) but doubles the consumption of sayha grace.
If you party you consume more sayha grace, if you have and pet do dmg you consume more.
There is 30 sayha pots per day to buy for adena/ 50 sayha grace mini pots from daily, but if you level even moderately fast it wont be enough and you will need to get it from cash shop.