Imagine this scenario: Generating a new PO file for (e.g.) Portuguese for a not translated theme.
Already in the wordpress instalation there's the WordPress main and ThirtyThirteen PO files for Portuguese. These tow files already contain a lot of strings that are probably in the new PO file generated for the not translated theme.
There could be a routine where WordPress and other themes PO files for Portuguese are scanned and for equal strings the already made translations are suggested to be included on the new PO file for the not translated theme