Helps if you are young and flexible, as bending over low to reach the levers
on the down-tube is needed.. relocating the shifting function
to the end of the drop bar simplifies that, and you shift the lever while holding on
to the handlebar, rather than letting go and reaching down.

in the 80s I had downtube shift levers as the stock part, micro ratchet sun tour ,
once the knack was developed, a quick slap of the lever made one gear change.

Bike control when carrying weight in the panniers , had me converting to bar end shifting.
and then the road bike got a set , too, as aging made bending down low less desirable.

for a more upright posture , thumb shifters work well..
also a simple lever, less to go wrong..