What’s the purpose of products groups in a Shopping campaign?

Product groups let you choose specific products from your feed to promote within each Shopping campaign.

• Product groups are remarketing lists that you want to advertise in each of your campaigns.
• Product groups are predefined themes that you can select in each of your campaigns.
• Product groups are keywords that are defined by categories you can select in each of your campaigns.
• Product groups are used to specify which products from your feed you want to advertise in each campaign.

Explanation:
Rephrased:
Shopping ads rely on your product data rather than keywords and are structured around product groups instead of traditional ad groups.

Product groups allow you to select which products or sets of products to advertise within each campaign. If you don’t specify particular products, your entire feed may be eligible for your ads, which can reduce targeting precision and potentially lower campaign performance.