Fun fact, shirt-tails were originally designed as a means for men to wipe their butts with the tails after defecating during the medieval period in Europe when no other means were available such as when the men folk were out hunting rabbits or witches.
With that in mind, the standup-desk is more conducive to the "tucked-in" look whereas the sitdown desk is more appropriate for the "untucked" appearance. It's simply a matter of aesthetics based upon the functionality of the human body.
Jon 