To improve performance, catalog entry meta data is serialized and stored in a field for each catalog entry. This allows all meta data information to be fetched from a single field and de-serialized, which reduces the number of database calls and greatly increases performance compared to non-serialized data fields.
The SerializedData field should be updated after updating any catalog data through 3rd party sources. The SerializedData field is part of the CatalogEntryDTO (data transfer object), and will be cached based on catalog cache settings. Storing images and files in meta fields will increase the size of the serialized data, and increase catalog cache size.
From the Commerce 7.5 documentation:
http://world.episerver.com/Documentation/Items/Developers-Guide/EPiServer-Commerce/75/Catalogs/Guidelines-for-working-with-catalogs/