You can absolutely add raw chicken straight into the slow cooker. However, cooking the onion first and sealing the chicken breast adds extra flavour to the dish.
The caramelised onion adds extra richness and the lightly browned chicken pieces will add extra flavour to the chicken. Cooking the spices in the pan first will also help release their fragrance, making the end result extra flavoursome and delicious!
It is, however, a myth that browning the chicken first will lock in the moisture. Any meat releases juices while cooking and there is nothing that can really be done to prevent that.
Overall, if you don't want to do any of the pre-cooking then you can add everything straight into the slow cooker, and it will be absolutely fine, but the initial cook step will elevate your curry from good to awesome.