Q: "Is there an advantage browning the cubes of meat in a frying pan prior to putting it in a slow cooker to make a stew?"
A: It’s sometimes said that browning meat ‘seals in the juices’, but that’s not the case. What browning does is add flavour through a series of quite complex chemical reactions known as the ‘Maillard reaction’: the effect of concentrating the flavours at the surface of the meat, and making the meat brown.
Meat that isn’t browned tends not to have much flavour. In a slow cooker, because there is no evaporation of liquid, you also don’t get the depth of flavour that evaporation provides.
So if you want to get the maximum possible flavour from your slow cooker stew, brown the meat first. Then use a little stock or wine, or other liquid, to de-glaze the pan after you have browned the meat. Stir up all those brown bits from the pan and put all this into your stew mixture, too. You should notice a definite boost in the overall tastiness of your dish.