The water intake and the rim always seem to get cruddy
It's not that you don't clean. It's that a round head is too wide to go under the lip, so it rides over the top and leaves that line sitting there. The grooves at the base and the drain go the same way, and it was always difficult to get them clean. Then whatever the bristles do pick up stays threaded in them, back in the corner, looking a bit skanky.
The angle is what gets you under the rim
Tip the head up and it goes under the lip and runs all the way round, then down into the nooks and crannies and the drain, without you needing to do any handwork. Most people are a little shocked the first time, not at how clean it comes up, but at how much the old brush was missing. The soft TPR nubs do it without marking the glaze, and the 16 in (41 cm) handle keeps your hand well clear of the water.
It doesn't get gross, and nobody notices it
Nothing threads into the nubs, so one flush rinses the head and a few flicks shake it dry. No fine mist spraying back at you near the rim, and no bristles sitting there to hold odors or go mouldy by next week. Then it goes away: on its no-drill hook if you took that version, off the floor and out of sight, or standing in its holder if you'd rather keep it by the bowl. Either way, most people using the bathroom don't even notice it's there.