Sorting laundry before washing is really about preventing four different kinds of problems that would otherwise happen if everything went into the machine together. Looking at it through those problems clarifies what to keep in mind.
Preventing colour transfer: Dark or brightly dyed garments can release colour into the wash water, especially when new, so keeping them separate from whites and pastels avoids permanent staining of lighter clothes.
Preventing fabric damage: Delicate fabrics such as silk or lace can be torn, snagged or roughened if washed alongside heavier items like denim or towels with zips and buttons, so separating by fabric weight and delicacy protects the more fragile pieces.
Ensuring heavily soiled clothes actually get clean: A few very dirty garments mixed into a load of lightly soiled clothes can mean the dirty ones don't get fully cleaned, while also transferring some of that dirt onto the cleaner items, so heavily stained clothes are best washed separately or pre-treated first.
Following the correct wash method for each garment: Care labels specify water temperature and cycle type, and mixing items with very different requirements, such as a hot-wash-safe towel with a cold-wash-only blouse, forces a compromise that can shrink or damage one of them, so grouping by label instructions avoids that.
Therefore, before washing, clothes should be sorted with an eye to preventing colour transfer, protecting delicate fabrics from rougher ones, ensuring heavily soiled items are properly cleaned, and following each garment's correct washing instructions.