To answer your main question (Why would you want to add potting soil to the ground soil vs. compost?), I should first explain that the potting soil I use is more properly called "planting mix." Years ago, Annie from Annie's Annuals said that she always uses planting mix rather than potting soil. I've followed her advice ever since. It's much cheaper than potting soil and it works better in the ground. As for using compost instead, I've had good results using compost as top dressing, but not as a planting medium.
The reason that I have to use something in addition to garden soil when I plant a band rose or any other plant is the Arum that plagues my garden. I think I've told you that this horrible stuff has spread to every part of my garden, driving me nuts by obscuring the shorter plants from October to March or April, when it mercifully dies down for the year. When I dig a hole to plant something, I take the opportunity to remove all of the Arum bulbs from the soil I've removed. Once they're gone, there isn't enough garden soil left to refill the hole unless I mix it with something else.
Here's a photo of the mess the Arum makes of my garden. There's an azalea hidden by it in the top left corner. You can only see one of the blooms. And a new little camellia is trying to push a bud above the Arum in the bottom center.
When I see this stuff for sale on websites, I can't believe anyone would pay for it. Mine came from a houseplant one of my neighbors mistakenly planted outside. Every house in the neighborhood is now overrun by it.
