[Pandamasque]

The only situation where a Safety Car is needed is a situation that calls for grid to be bunched together in order to allow greater intervals for marshals to not worry about cars passing through. And it's a rather narrow set of parameters that would necessitate that. I'm talking safety-related, not manipulating competition. When the track is so covered in debris that any clean-up work would be unsafe with cars moving through at any speed at all. In that case Code 60 is implemented immediately, then a Safety Car comes out and leads cars single file on the narrow path through the debris, while all of workers step aside and only resume work in the long intervals between the entire field passing though. That is the ONLY safety-related reason to have a safety car. One example would be the aftermath of that restart pile-up before turn 10 at Petit. But one could argue that was really a red flag scenario!
And yes, there is absolutely no reason why those course vehicles cannot go out on track and shield the marshals and medics under Code60 just like they do under SC. None!

In every other situation an SC is
1. just an extra vehicle that may also run over a marshal
2. disruption to race flow/strategies
3. constant cause of mayhem in the pitlane which regularly results in accidents between cars with mechanics narrowly escaping life-altering injuries
4.* a tool to spice up the show through all of the above, and to have an opportunity for broadcasters to mention sponsors, such as: sponsor of the pit-report, fuel provider, tyre manufacturer(s) etc., to insert an infomercial segment, to have an ad break etc.

*the real reason they are still used. Luckily IMSA is only guilty of some of those things, but nowhere near as bad as Indycar, which I find completely unwatchable despite the racing being very good.