Complete uniformity does a lot of things. If nobody stands out for any reason whatsoever, it creates a sort of equality. No one is better than someone else. No one has more than anybody else. No is happier or sadder, etc. People are kept artificially young looking, they take drugs to help control their emotional status, and a whole host of other things to help ensure societal uniformity.
From a law and rule perspective, having everybody be the same makes it so that anybody who stands out and is unique is easy to spot. That's why Lenina reacted so disgustedly to Linda. Everything about Linda's appearance screamed outcast. Linda knew it too, which is why she wanted her soma back. John stood out to Lenina as well, but in a different way. Physically he looked like he should, in fact he looked really good to Lenina, but John didn't want to immediately sleep with her. That made John stand out as well. So if you are trying to keep the order of law on this kind of society, any kind of individuality is easy to spot, keep track of and discipline or exile.