Why persist session information to the ’config.system.sessions‘ collection?

I don’t know why session information needs to be persisted every 5 minutes. Is it to reduce memory usage? And why it is a range collection, not a hash collection.

Starting in MongoDB 3.6, the config database contains the internal collections to support causally consistent sessions for standalones, replica sets, and sharded clusters and retryable writes and transactions for replica sets and sharded clusters.

But is it essential to store session information through ‘config.system.sessions’ collection?