Object caching is not page caching. Page caching stores finished HTML where as Object caching stores reusable results.

This can be Options, Menus, Metadata, Query results, or Computed values. And this is important because not every request can be page cached.

You see, Logged-in users still need performance, WooCommerce carts still need performance and Dashboards still very much need performance. And it's in these places that object caching earns its place.