T-Shirt Sizing is a technique used for relative estimation and high-level sizing of items. You use this technique of relative estimation as opposed to absolute estimation when you just need a rough estimate or comparison of items. Speed is valued over accuracy which stops people from overthinking and overanalyzing, as you just want people to use their instincts and gut feeling. Information may not be available at this point anyway for detailed estimates. It is therefore used early on in agile projects or when there are a lot of items to estimate in a relatively short time.
** Note: If you need detailed estimating and sizing such as in Sprint Planning, it is better to use Planning Poker or other detailed estimation techniques.