RICE Cereal...? Opinions and thoughts

SB
Just wondering what everyone's reasoning for rice cereal is? For baby to sleep longer? Iron supplementation? Just curious why everyone thinks this is a good "first food" to start your baby off with.
My mother in law fought me so hard to get me to give it to my daughter as early as 6 weeks because "she did with all of hers". 
Just curious as to why you think it's necessary or if you all do.. and what your opinions are on rice cereal! I have my own opinions on it and am personally against using rice cereal. My MIL reasoning was to get baby girl to sleep longer. I'm a stay at home mom (as was she) and have no reason to not be waking up when my baby calls for me and can (thankfully) make up for sleep sometimes during the day. I don't agree with giving it to her for that reason. If I were working my tune may change but even then I'd prob stick with barley cereal. 
How do you all feel about it, and have you researched it / and or the negatives of rice cereal at all?