Oк. Let's wait who knows for sure)
What have been said by both Omega and HCP is correct.
If you tick the "Final" checkbox, the transaction will be broadcasted without the RBF flag. Therefore, you will not be able to bump the transaction fees if it gets stuck (btw, right now 150 sat/byte is the recommended fee for a fast confirmation).
And another question in which case it may be necessary to set it so as not to raise it any more
I can't think of any practical use case for this unless when sending a payment to a service/merchant that accepts transactions with zero confirmation. They usually reject RBF transactions and do not accept the payment unless after 1 confirmation or more.