Cutting in is the part of painting that decides whether a room looks finished or looks attempted. Everything else is coverage. The line where the wall stops and the ceiling begins is at eye level for the whole life of the room, and a wobble there is the first thing a visitor's eye lands on, long before anyone notices that the roller left a light patch behind the door.
People choose colour for weeks and sheen in the last thirty seconds at the counter, and then wonder why the finished room does not look like the sample card. Sheen changes how much light bounces back at you, and light bouncing back at you is most of what you perceive as colour. The same tin, applied flat and applied at a higher sheen, will read as two different colours on two walls of one room.
It is a common and demoralising experience: the first coat goes on and looks encouraging, the second goes on and the wall is suddenly covered in streaks, patches and visible laps. The instinct is to blame the paint. Usually the paint is fine and the second coat has simply revealed what the first coat was hiding.
Sanding between coats is not about removing material. It is about knocking down the grain and the dust nibs that rose into the last coat so the next one has a flat surface to sit on. Once you hold that idea, the technique follows: light, flat, fine, and quick. Almost every failure comes from treating it as shaping rather than smoothing.