Object caching involves storing database query results so that the next time a result is needed, it can be served from the cache without having to repeatedly query the database.
As a content management system, WordPress is naturally—and heavily—dependent on the database. As such, database efficiency is crucial to scaling WordPress.
If you run a high-traffic site and requests to your pages generate a large number of database queries, your server can quickly become overwhelmed, in turn negatively affecting your site’s performance.
So when object caching is enabled on your site, it can help ease the load on your database and server and deliver queries faster.