You will find that most t shirt screen printers aslk for vector art . this does not mean it is the only or best format for tee shirt designs. Just for Silk screen printed tee shirts.

 The reason why is that the final production art or Film positives will come out with exacting clean detail.

The silk screen process for apparel ar garments uses Butt to Butt Registration as compared to a Trap or chock or overlay of color. This is where each color will butt up to each other exactly.

Halftones, or gradients, screen tints or percentages, will have a clean sharp edge unlike a bit-map halftone which gets the Dot pattern as an edge.

Then there is type.... with a photoshop file using halftones the text will also get this ragged edge as the Post script type does not rip over in Spot Channels.