Great question. 

Data conversion is for moving tax data files from another program, like TurboTax, to ProConnect. https://proconnect.intuit.com/community/convert-data-files/help/proconnect-tax-online-conversion-inf...

Batch Actions | Transfer Prior Year Return Data moves ProConnect tax returns from one year to the next.  

https://proconnect.intuit.com/community/customer-lists/help/how-do-i-proforma-or-roll-over-my-prior-...

Which tool do I use to transfer and convert prior year data from Turbo tax to Proconnect? 

The first link for data conversion. 

Do I need to do this every year for every client?

Batch Actions | Transfer Prior Year Return Data moves ProConnect tax returns from one year to the next.  

What is the use case of doing this ? Tax planning and comparitive analysis of tax years I am guessing. Is there anything else?

Do I need to do this every year for every client? Generally, yes. 

Batch Actions | Transfer Prior Year Return Data moves ProConnect tax returns from one year to the next.  

What is the use case of doing this ? Tax planning and comparitive analysis of tax years I am guessing. Is there anything else? No, use the COPY return to create a sample to test or plan. 

Once I use the "Batch Actions | Transfer Prior Year Return data" tool, then does it automatically create a copy return?

Like, are you saying I can transfer 2020 return data ( after e filing) to 2021 return, and then I can change numbers on the copy return for 2021 to extrapolate what the taxes will be for 2021 and this will help in tax planning.   

Does the software automatically create a copy return? How do I create a copy after transferring the prior year return data?