Motor oil 'detergent' helps keep dirt particles suspended, so they can be removed by the filter, instead of allowing them to settle and 'plate' out on components.
I ran the old Valvoline racing oil (before it became known as VR1) for many years, and it worked very well for me. Engines were very clean, so the detergent level appeared to be adequate for the job.
Castrol OTOH always left a layer of sludge in the bottom of the oil pan and in the rocker shaft (although not nearly as bad as some other oil brands).