What is "Date Joined Group" used for?

SOLVED

I have people who have transferred from another company to ours with their years service. Surely I should use "date Joined Group" that will automatically update their years service? 

No one has been able to explain to me what the date is used for.  Common sense says it recognizes the person's service thus leave, years service and any other recognition around length of service?

Thank you

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