Could you please once confirm on which version of Pega you are working.
If you are working in Pega 6 versions, you should have come across "Data Table" to hold the instances for Internal Pega table. It can also be mapped to external table via class mapping. In Pega 7, this have been modified to create a data type and add local storage for the same.
From RDB table, I think you are talking about creating a new table and then mapping it through external databse mapping wizard. Please correct me if I am wrong.
In Simple words, Data type is just a class type which contains data instances. It supports automating several processes: creating new properties,importing/exporting data instances, create db table instances and map to Pega Data, create data pages.
Because of these things we mostly prefer Data type.
Imagine that I am working ticketing system application, so what will the primary data type for my application : Support Request, Accounts etc. Hence I will create data types for these things so that it is easy for me to import/export data , adding property related to these things.
Please let me know on further queries if you have. And also more specific what you want to achieve so that me or anyone else in the community can help you better.