The slow cooker cannot brown the meat, so it can look gray and unappetising after cooking, if you place raw meat in the cooker. If you are in a rush you can do this if you prefer. More importantly, searing the meat starts to bring it to temperature so that the cooking process starts straightaway. Searing the meat also has another purpose. When you fry or sear the outside of meat at a high temperature browning occurs, which is the amino acid and a reducing sugar combining to produce different flavours. This is known as the maillard reaction and it's what makes the meat taste so good! The sugars also start to caramelise, which is another part of the flavour process. With all this going on, it's definitely worth an extra few minutes for the flavour!