A PIM gives one place to hold everything needed to sell a product: identifiers like the GTIN, dimensions, ingredients or materials, compliance fields, copy and images. Supplier data arrives in it, merchandisers enrich and approve it, and it publishes the result to the website, marketplaces and retail partners in the format each one requires.
Without one, product data usually lives in spreadsheets passed between buyers and the ecommerce team, and errors are found by customers. A PIM is not always necessary for a small catalog, but some form of checked, single source for product data is, and the onboarding of supplier data is a common place to add automated checks.