Why refresh lentils before making lentil soup
It's been years since I've cooked lentils, and I haven't done it very often, but I'm going to guess that the issue is that like other legumes and grains, you can end up with a gummy exterior, so by rinsing it in cold water, you both stop the cooking process and rinse off any starch that might've been over gelatinized.

If you hadn't done it, the most likely difference would've been a a thicker soup, and possibly overcooked mushy lentils.  (I don't know how lentils behave when colled; some starches will behave differently if you cool then reheat it vs. if you keep it warm the whole time it can get mushy)