system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is an identity graph and why are companies building them?
gpt
An identity graph is a collection of identifiers and their relationships to real users. These identifiers could include your email address, third party cookies and other stable pieces of data.

Companies want to buy these because they help make a profile of a person and tie together the data related to different identifiers.