I saw that cooked rice has 30g per 100 g of carbs and potatoes 15g. What is the advantage when bodybuilder eat potatoes?
White rice is a 19th century product. It’s a refined carbohydrate of low nutrient density. It’s so lacking laws require manufacturers add back synthetic vitamins to prevent frank malnutrition syndromes like beriberi. The only advantage to white rice is a shelf life of 2 years.
After milling and polishing, white rice has no bran or germ. These are co-nutrients needed for it’s healthy digestion and metabolism.
Brown rice is only hulled and is still a whole grain with the bran and germ, much healthier food. Brown rice has a shelf life of 6 months.
 
One unskinned potato provides:
One medium potato contains:
https://www.webmd.com/diet/health-benefits-potatoes
I once read that if you were to rely on an absolutely one sided diet (eg eat only brokkoli for one year, or only pasta) then potatoes would be one of the best choices because apparently you could theoretically eat only potatoes for one year without suffering from malnutrition.
Potatoes can have a bunch of different textures so you won’t get bored even if you eat with a healthy or low fat preparation. You can season them any which way. They are eaaasy to cook. They (used to be) cheap. Mentally, they are good for someone who is watching their diet because of these reasons, and body building is #1 a mental game - so I’ve heard, I’m more of a lifter ;)
Then all the nutrition benefits others are saying