Breastfeeding mamas I need advice

My Lo will be 4 months on the 28th, her 4 month appointment was today and her pediatrician suggested she start on rice cereal because she’s 15 lbs and they’d like her to be bigger. She says 15 lbs is like the median rate for babies in her age group, so she’s not below weight or above, just average. She’s exclusively breastfed and feeds on demand. I remember reading somewhere that rice has no nutritional value so I don’t understand why I would give it to her. Also she told me she should start eating baby food but is that even necessary??? Isn’t the breast milk sufficient enough??