If you buy just (premium/midi) styles you do not need a flash memory card. If you buy expansion packs with styles and voices you do need one. Same for audio style packs. But are you sure your T5 doesn' t already has one installed?
T5's that are 2nd hand often have one already installed but emptied/fromatted sometimes by either the previous owner or the dealer that sold it to you. I know my dealer asked me to bring my T4 back to factory default before I traded it in for a T5. Same thing perhaps happened with your T5 by a user that upgraded for exampel to a Genos.
regards,
Peter