It is mainly due to the Advertising ID. A not well known and not really documented problem with AdMob is the number of fraudulent and excessive clicking of ads that occur - this was learned of AFTER google bought admob for a large sum of money. A big shame for the early advertisers on Admob that more or less got robbed of some of their money.
Now Google, is trying to right the ship as fast as possible through these updates that will fundamentally help track that a click is actually a click and not just a case of spam or bots. To be perfectly honest, I think it’s a huge step in the right direction albeit very annoying. But we need this kind of transparency in order to grow as a community. I suspect when August 1 hits, there’s going to be a huge revelation of actual performance on mobile advertising and unfortunately some may lose significant revenue.
But it’s a necessary step in order to really take mobile advertising to the next level. We need to really start bringing value to advertisers rather than a lot of useless traffic.
thanks for the really helpful comment and explanation. I guess in the long term this would be better for everyone, however it’s really just a pain to constantly have to update apps because of these required SDK updates.