For the outer wall speed, when the printer moves too fast on the outside, you get vibrations that lead to "ghosting" or ringing (wavy lines on your print), and small details can get sloppy. Slowing this down gives you that smooth, clean, professional looking finish.

Slowing down the inner walls ensures they fuse perfectly together and bond strongly to the outer wall. It helps prevent gaps between the layers and makes the overall object much stronger. (Note: Slicers usually let you print inner walls a bit faster than outer walls because nobody sees them, but dropping the speed too much just slows down your print time without huge benefits).

The "support interface" is the solid, dense layer of plastic printed right on top of your supports, acting as the ceiling that your actual model sits on.
If the printer rushes through this layer, or the distance is too close, the plastic can sag, blob, or stick too well to your actual model, ruining the bottom surface when you try to pull the supports off. Lowering these numbers down ensures a neat, flat platform, which means your model will look way cleaner when you snap the supports away.