The landing lights turn off after flap retraction because they create drag and at high speeds there is no need to have them out during climbout.
Clearly "no need" while the rest of the world seem to think it is a good idea to be seen in the busier lower levels up to FL100, esp in London and other busy TMAs...And FL100 was always sop on the Boeing - whatever changed the industry-wide requirement for visibility. Safety over a miniscule amount of drag - no contest then.
In some situations they come out to help us descend quicker, if you're left high, the lights add some drag that helps get down or slow down quicker.
I'm speechless! Using landinglights to assist descent! Whatever next? Open the doors? Stick your clipboard out of the wndow? What happened to the mistake handle if you've misjudged your approach planning (or had it misjudged for you)? As if half a square foot of landing lights is going to make any difference at all? Bejasus!
Yet more examples of accountant-led dogma being unquestioningly swallowed at the expense of common sense and airmanship.