Browning ingredients (both meat and vegetables including the aromatics) before doing a braise or stew (which is what slow cookers do) helps develop depth of flavor, through the Maillard reaction where proteins and carbohydrates react together to create a myriad of flavorful compounds. Vegetables that are high in sugar, such as onions or leeks, and even carrots may also have some caramelization, where sugars react with each other, again creating flavorful compounds.
Especially with beef, these deep browned flavors are often what people associate with the product, and what they expect to taste.
On the other hand, browning almost by definition overcooks the outside of meat well past well done, so it is somewhat drier and tougher, although this can be mitigated by a long braise Some experts recommend browning only on one side of cubed meat, to compromise between getting flavor development, and getting good texture.
The one thing browning or searing doesn't do is "seal in the juices"; that is a myth that is well de-bunked.
The choice to brown or not brown is one of taste and balance. It is traditional in many recipes, especially of Western European heritage. There are many traditions where browning is not as frequent, including true Mexican cuisine and many Asian cuisines.
Choose what seems most appropriate and tasty to you in a given dish. Personally, I like the flavor development, and almost always opt for browning.