Why does a BC-304 need a shuttle or ship's boat when it already has transporters and rings?
Short answerTransporters can be jammed or outranged; an organic shuttle independently recovers personnel, moves cargo, and connects ship, planet, and installation.
NotesA commenter notes that transporters and rings do not eliminate the need to move people or cargo over longer distances or under jamming. Captured Goa'uld cargo ships offer a possible stopgap because several allied groups already operate them. Their availability makes the failure to develop an organic Earth equivalent especially conspicuous. A shuttle would preserve independent movement between a ship, planet, station, or nearby gate when fixed systems fail. Optionally crewed F-302s and more imaginative use of the hangar ceiling could also increase the ship's aviation capacity.