If you like your sausages to be browned when they are cooked and ready to eat then you should brown the sausages before putting them into the slow cooker.
But if you don't want to brown the sausages before slow cooking them you don't have to brown the sausages before slow cooking them.
Raw meat including raw sausages can be put directly into the slow cooker without a need to brown or cook the meat or sausages beforehand.
Just make sure you thaw the sausages out first and any other meat before you put it into the slow cooker so it cooks properly.
The purpose of browning sausages before putting them in the slow cooker is to make them tastier and make them look nicer when they come out of the slow cooker.
But you're not required to brown or cook the sausages before slow cooking them.