As jaisoni_trp said, they are used by the Monitoring Apps in Qlik Sense June 2017 or higher. Ever since Qlik Sense 3.x, the monitoring apps have used data connections which tap into QRS API endpoints to add additional information / more efficiently get information rather than looping through a bunch of TXT files on disk.
They also are useful templates to build out your own applications using the QRS APIs, e.g. determining when Login Access Passes will expire: login access pass full - YouTube
I think they can be deleted. I found as below in help:
"When you upgrade from Qlik Sense3.2.x to June 2017, or later releases, eight new REST data connections are introduced (monitor_app_REST_). The existing qrs_ data connections will no longer function due to functional changes to the REST connector. These qrs_ data connections therefore can be removed."