There are different types of CSS partners. Some offer a "self-service" and others offer a "full service" where the campaign control and bid management is completely taken care of for you. Google recommends that you work with different types of CSS to maximize the opportunities within Shopping Ads.
A "self-service" CSS partner allows you to migrate your existing campaign from Google Shopping to take advantage of cheaper CPCs, although the support you receive may be limited. A fully managed campaign is usually complementary, so in addition to your direct campaign (managed internally or by an agency), CSS will go into the auction on your behalf to provide you with greater reach. This will bring your shopping ads more visibility, clicks and sales.
A full-service CSS partner will use other bidding strategies to achieve your goal. Some will use Google automation tools like Smart Bidding or Smart Shopping. Others, like Connexity, use their own homegrown bidding tools. So alternative strategies can extend the reach of your own campaigns.