Here’s the idea: mom, dad, kid, baby. The kid is tall enough to ride but the baby is not. Mom, dad, kid get FP for the ride. Mom and kid ride, dad stays with baby. Then they swap so dad rides with kid, and mom stays with baby.
Without rider swap, Someone in that scenario would have to ride alone. With rider swap, everyone gets to ride with a family member, there’s always someone to watch baby, and as a bonus the kid gets to ride twice. That’s the basic idea behind rider swap.
You could not do this with FP alone, because, say, dad and kid ride while mom waits with baby, that leaves mom alone with a FP and no one to ride with.
Technically, everyone in the FP queue needs a FP, whether they are doing rider swap or not. Is it the case that sometimes CMs allow someone to “sneak in“ for one reason or another? Yes. But that is not something you should count on.
I don’t know why, but there are some things at
Disneyland that seem to have very ill-defined rules. You’ll be told one thing on the phone, but then day to day, or even attraction to attraction, the rules are different. FP Rider Swap combo is one of those things. YMMV.