Invoices can be modified post booking in a variety of ways... such as changing rates, adding or removing fees, taxes, etc.
There may be times where you would want to restore that invoice to its' original state. The regenerate invoice button will reset all rates, fees, etc. to the values that were in place at the time of booking.
Before regenerating an invoice, it is usually a good practice to print the invoice or save it as a pdf in the event you need to reference what was on the invoice prior to regeneration.
Note that invoice regeneration will not remove any transactions (payments or refunds) applied to the invoice.