Obligatory XKCD Reference: Standards
Simply, you want to arithmetic on the values.
Addition, subtraction, multiplication of numeric values that represent monetary amounts in different currencies.
If those values are "buried" inside String values inside a single database field, then you have to waste processing time pulling those values apart so that you can "get at" the numeric part and then do your arithmetic on that.
Databases are really, really good a finding bits of data and putting them together.
They are [all] generally rubbish at finding big chunks of data and pulling them apart again.
This is a case where the storage representation of these values (two fields, one numeric value, one character currency code) is different from the way that you or I might choose to think about them.
That representation - the one that we would use - should be delivered by the Application, interpreting what's stored in the database into what we are used to seeing (and reinterpreting values going the other way, from what we use into the database).
Regards,
Phill W.