Time Dimension
row_id is used to facilitate the Time Dimension for entities that support it. By having this additional identifier, it allows an entity (product, category, sales rule, etc..) to be stored more than once in their respective MySQL table but expressing different values for the same entity. These altered versions of the same entity are created using Magento's "Content Staging" feature.
Content Staging
You can schedule changes for sales rules, category, products, etc. to happen during a given time frame or permanently. row_id is the one of the key pieces to allowing this to happen as stated above, allowing those same entities to have different values at different points in time.
Example: You want to change a product name for a month. You can schedule that change using the content staging by specifying a different product name for the date range needed. Behind the scenes, Magento will utilize that row_id (among other meta data to support this feature) to store the scheduled change. If a customer visits the site during the time the product name was scheduled to be changed, Magento will end up loading the row_id corresponding to how the product was scheduled to "look" at that point in time.