Why is there a card deck with T-Shirt Sizes?

Some teams prefer to estimate effort in a relative way using t-shirt sizes. 

For example, the "s" card represents a story that requires a relatively small amount of effort to complete. The "xxl" card represents a story that requires quite a bit larger amount of effort to complete. 

Try this: before backlog refinement and prioritization, have the team estimate the upcoming stories using the T-Shirt size deck. This will help the Product Owner understand how much relative effort is required per story, and s/he may be able to make informed decisions based off the relative T-Shirt sizes given.