I am either the cause or effect of devastation
Drop a letter, I may serve, judge or contain.
I'm not 100% satisfied with this, but perhaps you are
a COUP, which (1) might well be the cause of devastation as the new regime eliminates its enemies and (2) might well be the effect of devastation as such things often happen when a society has been destabilized by e.g. losing a war;
and removing a letter might yield
a COP who both serves (the public) and judges (criminal guilt, although of course others have the last word on that) and a CUP (which contains liquid).
-- OR --
(I'm not 100% satisfied with this one either) you might be
a REEF, which might cause a shipwreck or be formed by one
and losing a letter would yield
a REF(eree) who both serves and judges, and a REF(erence) as in computing which kinda-sorta contains something else. (This last one isn't very satisfactory since usually you want to distinguish between something that merely refers and something that actually contains.)
It could be
Devastation can be caused by the value of something going down or
Devaluation of money can be caused by devastation
EVALUATION can be a judgement of something, or contain a judgement...Not sure about serve though.
EDIT - I suppose an evaluation could serve as a tool by which to use as a plan for improving things.