I would rate it as 'desirable' rather than 'necessity'.
-You could muck it up, but usually you are returning to zero under less pressure than when you were 'dialling-up', so you can be a bit careful.
-many scopes have turrets that can be reset to zero, so you only have to turn back to zero each time. (as opposed to true "zero stop" which is a mechanical stop at the same position).
Hope that helps.
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