It is well known that wood expands when it gets wet. Unfortunately, this can also happen the first time you paint. And when it dries again, individual fibers often remain standing longer. This can have a very disturbing effect and must then be smoothed out extra.
If the surface is to be stained it is especially bad because sanding again removes some of the stain. It is therefore more difficult to achieve a uniform color tone.
But even if it is varnish, a thorough intermediate sanding must be done before the next coat of varnish.
The effect is particularly noticeable in softwood (e.g. spruce). It's not as bad with hardwood. Here you can possibly omit dewhiskering altogether.
Therefore, it is customary to dewhisker once before the final sanding. I.e. you go over the surface once with a water-damp sponge or cloth and let it dry (one hour drying time is usually enough). Then the final sanding takes place.