Browning meat before slow cooking the meat is not required but when you brown meat before slow cooking it gives the meat a more rich flavor and makes it cook faster as well.
But you don't have to brown the meat before putting the meat into the slow cooker but some people find it worth the effort to brown the meat before slow cooking it.
That is for the richer flavor mainly but the meat will cook just fine if you just put the raw meat directly into the slow cooker.
Just make sure you allow the meat to cook long enough in the slow cooker so that it's fully cooked but you can cook meat in the slow cooker without browning.
But if you want a richer flavor to the meat then brown it before placing it in the slow cooker.