Breast shells were once used to help flat or inverted nipples stand out more. But research showed they do not help. Other devices may be more effective. If you have flat or inverted nipples, consult with the International Board Certified Lactation Consultants (IBCLCs) to get suggestions on what can help in your specific case.
You may decide to use breast shells if your nipples are sore, to keep your bra from rubbing them. If you do, be sure to apply an ultra-purified lanolin, like Lansinoh or hydrogel dressings, to keep your nipples from drying out. Also know that injured nipples are not a normal part of breastfeeding, so if your nipples are sore, get help.