They have a bunch of cool little apps (that are free) like BlockBlock that let you know when things are happening you wouldn't have otherwise allowed.
For example, BlockBlock warned me randomly about 30 minutes ago about an app that was being silently installed in the background.. something I hadn't seen before called MRT.app.
Turns out - that was Apple silently updating the OS to protect against Zoom. Wouldn't have known if it weren't for these apps.
The "nice folks" at Objective-See is Patrick Wardle, a former NSA rootkit expert who would like nothing more than to install various close-sourced components on your computer.
Funny how both companies have "objective" in their names.