PFD goods are the starting point for garment dyeing and tie-dye. Normal finishing adds softeners and optical brighteners that make white fabric look crisp and feel slick, but those finishes can stop dye from soaking in evenly. PFD skips them, so the fabric is clean, absorbent and ready for color.
In the hand, PFD tees are often a creamy, natural white rather than a bright optical white, and they can feel a little firmer until washed. They have not been through a hot dye bath yet, so they can shrink noticeably when dyed; allow for that in sizing if you dye them yourself.
For merch, PFD matters if you plan custom garment dyeing or tie-dye, or want a natural off-white shirt rather than a bleached white. Ask the dyer or printer whether they have worked with that exact style before. See the garment-dyed blanks guide.