In-app cash purchases
I disagree that it was not necessary to buy cash in IB 1. It becomes increasingly necessary, if you actually want to make any progression, the further you get into the game. Your character only increases in XP if he hasn't mastered all the items he's currently using. The more mastered items you carry the less XP you get. I'm currently around level 50 and I spend half my grind-time without a single item to level meaning that I'm getting no XP per kill. The other half of the time I usually have only 1 item to level meaning that I get a fifth of the maximum XP. This is all because the cash drops don't scale up at the same rate the item prices do. Levelling becomes increasingly arduous teh further into the game you get unless you actually pay for some extra gold with your real hard-earned.
They sucker you in with several hours of easy levelling with no cash issues and then hit you with an increasing need to spend real cash in order to progress further.