We know that infection is by hands and 'front' (hence plastic apron); underarm means no infection.  Covid-19 doesn't live well on fabric and this can be wiped down and washed.

It provides an easy way to carry important personal kit (access card, bleep, phone, pens, patient lists, wallet, keys) on a person without need for a locker.

It prevents items dropping (which happens from the pockets of scrubs).  And scrubs have too few pockets.  It keeps these items out of the way of patients.