The more popular (and difficult) cousin to Daily ARPPU is Daily ARPU, Average Revenue Per User. ARPU measures the average amount of money we're getting across all players, whether or not they've purchased.
ARPPU increases if purchasers are spending more money. ARPU increases if more players are choosing to purchase, even if the purchase size stays consistent.
No one metric can tell the whole story. That's why it's so helpful to have many KPIs on the same dashboard.