T-shirt sizing is an important estimation measure agile teams employ in two important cases - firstly to size epics where the requirement quality is not fully clear or requirements are still not solid and secondly, in cases where the teams are new to agile or where they have just onboarded the agile bandwagon and have no prior experience estimating user stories. T-shirt sizing serves as a relative estimation measure helping team size items utilizing the principle of relativity and serves as an important yardstick for any epic/sprint planning or refinement session.