To low labor force, high efficiency.

It could avoid being infected by acin scratch, when the staffs open the cap of glass by hand.

If draw the cap by hand, just because of negative pressure inside of the tube, the blood maybe splash due to overexert. They should wash hand often, and will be easy cause cross infection.

It can be more actual for biology safety thus fundamentally eliminating rudimental gas inside tubem threaten protential biological contamination when open cap by hand.