Because correct breast shield size Matter.Silicone Flange Insert makes breast shield/flanges funnel smaller.Incorrect size of breast flange/shield can greatly affect in milk production that will also lead to soreness or clogged ducts.How to find the right size1. Measure the diameter of your nipple (Please see photos for guide on how to measure). The standard flange size is often 24mm but you may need a smaller flange/shield based on your measurement.2. Compare the diameter of your nipple to the size guide chart (Please see photos)Compatible with:Spectra 24mm (17-21mm Flange Insert) and 28mm (24mm Flange Insert)Spectra Handsfree Cups.Medela Manual or Electric Breast PumpYouha Electric Pump and Handsfree CupsCimilreHorigen (No Cutting Needed)Real Bubee (No Cutting Needed)RH228 (No Cutting Needed)Wisemom (No Cutting Needed)Generic/Unbranded Wearable Pumps