What is an SEO event?

A recorded change: a page rewritten, schema added, a redirect fixed, a section restructured, a batch of internal links repointed. It records what was done, where, and when.

The date is the load-bearing part. Without it, performance data and work history are two separate stories that can never be checked against each other. With it, you can finally answer the question that matters after doing work: did that do anything?