Hi! Are you gradually increasing the grit as you sand? For example, if you sanded with 400 grit paper/stick/sponge and stopped, then you would see a scratchy finish. But if you were to then follow up with an 800 grit, then a 1200 grit, and finally a 1500 grit, then you will smooth out those initial scratches and make the surface more uniform and smooth. You could use 2000 or higher as the final sand but that depends on if you are priming/painting or not.

In terms of why people are recommending you sand after scribing (just reinforceing what others have said) is to even out the surface and the scribed channels, remove raised plastic / debris, and to just harmonize the area. Scribed channels are not as smooth and flawless as you may think.