The question is an interesting one because different gems are cut into cabs for different reasons. In many cases it is because the gem has special properties that are displayed only when it is cut as a cabochon. Examples are gems that display optical phenomena, including asterism (the star effect), chatoyancy (the cat's eye effect), iridescence (e.g., opal) or adularescence (e.g., moonstone). Most gems that are opaque rather than transparent are cut en cabochon rather than faceted.
High quality material with excellent transparency will almost always be cut and sold as faceted stones, especially when it comes to sapphire, emerald, ruby, tourmaline and diamond. You may see lower grade material in gemstones such as sapphire, ruby and garnet cut into cabs. If the gem material has very good color but is not sufficiently transparent or clean to be faceted, it can still be shaped and polished to be a very attractive cabochon.