Why would my mother need a POA when I am on all her accounts?

My 89 yr old mother lives with me because she is no longer able to live on her own. I take care of all her finances. She doesn't have a lot - only one bank account and a mutual fund account which I am on both accounts along with her. We are joint owners with rights of survivorship on a condo worth about $75000 which I am looking to sell. She is currently of sound mind and would be able to sign if I sold the condo. We have a living will for her which states the patient advocate if she is unable to make medical decisions. Is there any reason she would need a POA?