katello-agent and katello-host-tools have different functions, although katello-agent has a dependency on katello-host-tools to avoid some code duplication.
katello-agent, as the name suggests, places an agent on the client system in order to enable package actions (install/update/remove). The same can be accomplished with Remote Execution as you’ve read about. katello-agent will be removed in Katello 4.0
Historically, katello-host-tools was responsible for uploading the installed package and enabled repository information from the client to Katello to facilitate applicable Errata calculation. It still does this if necessary, but that functionality has been assumed by subscription-manager in newer versions which are available on CentOS 7+.
If you’re running the latest subscription-manager on your CentOS 7 (or 8) hosts then katello-host-tools is not required at all unless you are using katello-agent instead of REX.