I know I can't be the only one who's tucked (jammed) butter slices into the slits on an already topped pie... or forgotten it all together.
I was dotting away other day, when readying to host our Bible Study group for a nearly impromptu dinner by, among other things, making rhubarb and apple pies. It suddenly occurred to me that I'd never asked, nor ever researched the reason for doing so. I can't say I'd ever noticed the difference when I forgot, and I'd certainly never heard from my family, "It's obvious you left out the dots of butter, and we will not eat this second-rate pie!"
Maybe you already know why we're supposed to do this. Maybe you don't care. But today I searched the internet for the answer.
I have to say my findings are less than unanimous, or something about which I feel certain.

I found one seemingly scientific reason for adding butter to the pies. A contributor to Straightdope.com named "Flutterby" reported that a canning book she has says that the butter "helps to prevent cooked fruits from foaming." Not sure I've ever had a foaming problem with my pies, but I sure don't want it!
I think I will continue --when I remember-- to dot the pies with butter. Pie making, after all, is a time-honored art, long practiced by loving grandmothers who were amazing bakers... like mine were. And if it's what my grandma wrote in her favorite recipes, I'm going to do it.
Do you know of a more specific or confirmed explanation?