Brands use branded content to help meet the customer where they are. By partnering with others, who have their customers in their audience, they can reach those customers more effectively. Leveraging the work the partner has done to create a unique audience and tone of voice. It also means the brand can speak to the customer in the tone of voice created by that partner.
It is a really effective way to reach those audiences, where they are. It creates the opportunity for richer engagement over say a banner ad. So brands often use it in the media mix, to help engage customers and get cut through on key messages.