Prior to IDFA, advertisers used other unique on-device information (I believe they had access to the IMEA which was not changeable). With IDFA, users have the option of opting out or at one point resetting the IDFA on the device.
IDFA was fundamentally put in to give uses more control. It wasn't really enough, but it was an improvement over what was before it. Now Apple is improving it again by making it opt-in instead of opt-out.
Prior to IDFA, advertisers used other unique on-device information (I believe they had access to the IMEA which was not changeable). With IDFA, users have the option of opting out or at one point resetting the IDFA on the device.
IDFA was fundamentally put in to give uses more control. It wasn't really enough, but it was an improvement over what was before it. Now Apple is improving it again by making it opt-in instead of opt-out.