Where and when would a 737-700 be taking off at an altitude of 19 kft? Another question triggered by reading the 737 technical site; according to this section of the page on the 737's pressurisation system, the 737-700 comes with an option that allows it to safely take off and land at altitudes of up to 19 kilofeet AMSL (my emphasis):

High Altitude Airport Operations System
Is a customer option for operations into airfield elevations of up to 10,000ft (all NG), or 13,500/14,500ft (-6/700 series only). Changes include: Addition of high altitude landing selector switch to transfer to the high altitude mode, cabin altitude at warning horn activation is increased, an extra hour of emergency oxygen (bottles stored in aft cargo compartment), 10 minute take off thrust if engine inop; winglets, carbon brakes and fwd Cof G options are recommended. For the 737-700 the takeoff envelope is increased to 19,000 ft &amp; 0.50Mn. The option is usually taken with increased MLW, carbon brakes and RNP

Yet even the highest-altitude airport in the world - Daocheng Yading (DCY/ZUDC) in western China - is only 14,472 feet AMSL.
What justified providing the 737-700 with the ability to operate from a completely hypothetical airport over 4,500 feet higher than any existing in the world?
As Ron points out in the comments the question is not so much where as it is when. Aircrafts see the density altitude of the air around it not the true altitude they are at. On a very hot day at a very high altitude airport the density altitude may very well be higher than the true altitude by a significant amount which affects takeoff performance. The FAA provides a nice overview of density altitude here. SO even though there are no airports at 19,000 MSL a lower altitude airport that routinely sees very high temperatures can yield the same situation.