Degraded Battery Warning

I have a 2 year old SE (2nd gen). When I checked the battery status, the message indicated that the battery health is significantly degraded and the Max capacity is 86%. It is recommending replacing the battery. I have never needed to replace a battery in my iphones before. How critical is it that I do that? My sister has an 8 that is showing 82% capacity, but she is not getting the same message. Any info about this would be appreciated!