Translations are not applied in odoo intentionally. It allows you to change or create translations directly in webclient. update module won't override translations. here "Syncronize Terms" comes in picture when you want to sync terms you can use that wizard and it will update your latest translations. if you have multiple translations for same term it would update translation with higher priority

Note: priority for translation is taken from 'sequence' value from __openerp__.py file in module. default value for sequence is '120'.

Purpose of that action is to update translation fields with new fields that you potentially added in your module (or changed/override).

Potential cases: if you create new module and add new fields on some object, or create new .rml file etc.

For example: add field with string (name) on eng base 'technical_description' and after sync with french you will get that field in view so you can translate it