No advantages except that it makes an otherwise not allowed data model in Qlik possible.
Synthetic keys are the result of your data model having two (or more) tables with two (or more) field names in common. Qlik only allows a single field to be the relation between two tables, so it creates a helper table and a synthetic key to achieve this when your data model does not fit the rules.
The downside is that it is more resource heavy, can cause your calculations to go wrong and makes your data model harder to read, so we always advise to try and remove them.
In 99% of the cases removing a synthetic key is very easy:
- Rename fields that are not the same thing
- Remove fields that are double (as in "the same data in multiple tables")