That’s exactly it. It softens the outside of the cut-up potatoes, which then absorbs a bit more olive oil for the roasting part. I can’t tell you how long I steamed them - depends on the size of the pieces of potatoes. I think I did these large cut-up fingerlings for maybe 10 minutes or so? A fork went into the outside slowly but easily enough, and slowed down once the tines hit the inside still-uncooked part.
After par-steaming, I let them sit in the pan I’m going to use for roasting for about 10 minutes or so to slightly cool off, then drizzle the olive oil over top, sprinkle with s/p and whatever else I’m using, toss them carefully so they don’t break up, and then roast in my convection toaster oven, stirring every 10-15 minutes so they don’t stick and so all sides are able to get browned and crispy.