Dicing the chicken breasts allows them to cook through quickly and absorb more balsamic vinegar. Plus, it means when everything is done baking all you'll have to do is build your veggie bowls!
I do not use olive oil when preparing this sheet pan meal to keep it extra light and haven't had a problem with sticking for two reasons: (1) There's plenty of balsamic vinegar in this recipe so the liquid does help. (2) About 15 minutes into the baking time I move the chicken around to help prevent it from getting stuck. If you worry about the chance of sticking or have had any issues before with food sticking to your baking sheet then certainly feel free to add a drizzle of olive oil or grease your baking sheet!