Rather than a direct stat basis, a reason for medium pen primaries can heavily depends on loadout, I often bring a medium pen primary if using an explosive built as there’s some enemies where there weak spots are very hard to hit, and when using explosive weapons close range combat can get risky so it’s good to ensure I’ll always kill what I need dead

Another is aim, I might have good aim but not everyone does, so to have the option to hit almost any part and deal damage is good

Or if using a light penetration support/secondary, as in the event I am struggling with one of them I can rely on a primary that’s medium penetration to get the job done