I disagree! The butter greatly enhances the flavor moreso than keeping the jam on. If anything it allows the jam to slide off more easily.

Bread is coarse and crumbly. Jam is gooey and wet. Surely butter makes it less likely for the jam to stay on. I always assumed it was just for flavour. My grandma used to do butter toast with just a sprinkle of sugar on top, so I never even considered that butter was there for a functional purpose.

Butter on peanut butter is just weird, though, unless you're buttering before toasting. But then peanut butter is a recent import here, so maybe it's an old fashioned thing in the places that cultural imperialisted peanut puree unto us. Who's to say.