Most things are fine when executed by everyone, but there are a few things that should be handled by just the server, like scoring and AI behaviour.
Say you've got a trigger that starts an AI ambush when a player walks into it. Your trigger would be triggered once by West present, with the condition:
local Server AND this
And activation:
ambushergroup move getPos ((list triggername) select 0)
The trigger will never be activated by a client, and I assume that since the first condition is false that clients won't even bother checking the trigger, sort of an optimization.
Note the when a trigger is only activated on the server, any hint/titleText effects it produces will only be on the server. If you added hint text to the trigger's activation, only the host would see it. There are ways around this, read my tutorial about it.
Putting ?!(local Server) at the beginning of a script isn't the only way to make something server-side. (local Server) returns boolean true or false, so you can put it anywhere as a condition or as part of an "if" structure.