There's a ton of user data out there – data that is ideal for advertisers who want to target and serve very specific user segments. Web history, demographics, and other user-specific data turns each user's online footprint into a roadmap for advertisers. Trackers, via cookies, harvest this data – why not put disparate cookies together for greater clarity? The process of joining these datasets in real time is known as cookie syncing.
Definition: Cookie Syncing
(According software engineer Robert Heaton)
Many [user] trackers exchange user IDs through a process known as cookie syncing, an intricate dance unwittingly played out by [a user’s] browser. ...This process concentrates and centralizes knowledge about [a user]... and helps companies you’ve never heard of build more complete pictures... Once two trackers have synced cookies, they can share details on which websites a particular user has visited… [including] demographic and personal information.
As far as Ad & MarTech is concerned, more data = greater clarity = more effective targeting, less waste for advertisers, and higher CPMs for publishers. There are some very good technical reasons for cookie synching.
The tech problem: direct, in-browser cross-cookie access is verboten. What's more – each cookie uses an identifier for each user that is unique to each cookie. User matching between cookies requires a translation to match two (or more) cookie-based data sets to one composite individual, and rather than just swap data in browser, the process of syncing gets highly technical. What's more – during this process, third-parties get involved. A cookie synced with a third party is an example of data leakage known as piggybacking.