Add option for mods that can be loaded on the server, that are not required on clients.
Having serverside only mods is sadly a necessity.

Why 1: Cheating
Since arma reforger is a multiplayer game and has a notoriously weak anti-cheat: battleye,
it is not able to protect the game, in a similar way it(battleye) has failed to protected escape from tarkov.
Without a kernel level anti-cheat solution, such as FACEIT anticheat, the protection battleye provides is minimal.

One small, yet very useful thing that would help is having server-side mods.

Having serverside code be only available on servers would help a lot with making simpler code that is less likely to be exploited.
Since arma reforger is a modding platform, modders are less experience and writing bulletproof code is not easy,
while writing serverside only code would be much easier for inexperienced modders like me.

This came apparent to me when I was in a private dev stream of a recently released multiplayer FPS,
where the dev talked about how hackers have tried to call all random functions from the game,
tried to send requests to the server and so forth.
As well as the huge issues with cheaters/hackers that games such as the cycle frontier and escape from tarkov have been having.
It's a clear sign that having more methods and features to counter cheaters are necessary.

Why 2: Convenience
Having serverside only mod is easier in terms of not having to update clients for small serverside patches.
Having all of the serverside code available on all clients is just unnecessary.
Reforger as a modding platform will probably have all sorts of mods, as arma 3 has had.

Having the option to make a serverside only mod would be great, for having vanilla servers, with small modifications serverside.
Just the convenience is a nice option to have.