I remember that too! My grandmother used to do that and so did my aunt. There would be a thick plug of wax on top of the homemade preserves.
You got me curious so I went on a Google hunt and found that it isn't recommended to seal jars like this anymore because of the rate of failure in the seal that ends up exposing the preserves to bacteria and mold.
Thanks! I should have looked it up. Isn't it funny the way they did things back when our grandparents were little. My gram is no longer with us or I would have asked her. I am pretty sure she always sealed her jars like this (even as an adult).
I guess they made do with the technology they had available. I remember my grandmother's basement had a whole area with shelves full of different jars of things she made. She'd check them out regularly and a lot of times would find that a few burst. It's amazing more people didn't drop dead from botulism.