You would opt out, typically, if you wanted to reserve your rights to sue on your own behalf. You would almost certainly recover more if you sue for yourself, though of course you’d have to pay an attorney or find one who would take the case on contingency.
If you opt out, that reduces the size of the class, so the total award would typically shrink by some small amount.
If you don’t opt out but don’t collect the funds, typically the settlement agreement includes a provision about what happens (e.g. the money goes to a relevant charity).