I was like you, but after trying it for a few days, I can tell you that a mouse makes a lot of sense when using your iPad on your desk, with a keyboard. It’s much more convenient than raising your hand, also, certain actions like text selection are easier. There is also a third reason which is a bit more nuanced and well known to stylus users on desktop computers (and for Apple Pencil users). While the Pencil (and styluses like Wacom Pro Pen) are super-precise, they also register even the smallest of movements (as they should). So, when moving something precisely with them, like a slider or a transform node in a graphics app, raising the stylus causes a micro-movement that registers as an input. This happens because it’s really hard to move your hand in a direction that is exactly perpendicular to the screen. This causes the screen element to move slightly as you raise your hand and makes certain things hard to do with a stylus (like scaling an image with pixel precision in Photoshop). A mouse doesn’t have this problem as you can easily lift your finger from a button without moving the mouse. I actually find certain things easier to do with a mouse than the Apple Pencil in Clip Studio, for example.
So, these are some of the reasons. Mouse support on the iPad - even at this level - is really great.
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The pointer has a small dot in the middle which is the actual hotspot, just like the tip of the arrow on a Mac pointer. It is actually very precise.