Lean meat is harder to cook correctly. If overcooked, it gets tough and dry, whereas fatty meat can take more abuse. I can't count how many overcooked porterhouses I've had where the strip side was decent but the tenderloin was dry. That won't happen with sous vide, and it's much harder to overcook something underwater compared to using fire. Sous vide will let you turn out well-cooked lean cuts with little effort and not much skill other than pre-plate browning.
On the other hand fatty meats tend to taste better and are cheaper. These often have more connective tissue as well, making them consuming to prepare and less marketable than other cuts. Poor Italian immigrants would slow cook cheap beef cuts in broth to produce Italian Beef, which is very tender. Sous vide is similar to that process. It will liberate all the fat in the meat, where it can be discarded or incorporated into the meal as desired.
In short, for average cooks compared to other methods, sous vide will let you cook more consistent lean cuts than otherwise and cook fatty cuts with less effort like trimming, monitoring flare-ups, and flipping while cooking.