It's all pretty new to me, so in order to find out more about the pose I have consulted a few sources, none of which could be termed particularly authoritative. Nonetheless, there appears to be general agreement that the horaltic pose serves a number of purposes. The term itself may be derived from the name "Horus", and that's the designation of the Egyptian god of the sky, apparently associated both with the Sun and the Moon, but could just as well be a corrupt form of "heraldic" (the phrase "horaltic pose" easily outnumbers "heraldic pose" when fed into a search engine, but this is merely circumstantial evidence). Anyway, vultures, other raptors (various species of eagle are usually mentioned), storks, etc., assume the pose when fully exposed to sunlight.
A few speculative interpretations may be found. According to one of them, birds assume the pose mainly in the morning to, in a way, warm themselves up cheaply. Another explanation is that they spread wings to dry them. The most convincing one when it comes to the vultures in the Negev is that holding their wings in direct sunlight helps them to channel parasites into areas to which the birds have easy access. So the idea would be to get rid of parasites more easily.
This makes a lot of sense to me as in the few cases I've witnessed the behaviour was generally preceded by a feeding which - with tiny pieces of regurgitated content not finding their way directly into the chicks beak - would be conducive to the arrival of innumerable passengers. Flies - these "parasite transmitters" - can often be seen circulating close to the nest or even landing on the wings a parent vulture especially during a feeding on a sunny afternoon (and most of the afternoons are very sunny, indeed).
Our griffons spread their wings - at least as far as we can observe here! - only when exposed to sunlight, which is regularly in the afternoon, mainly between 1 and 4 pm local time. Unless they also assume the pose in the morning somewhere out of the nest, this rules out the need to warm up (the "solar sails" hypothesis, as it were) before taking off. Mom K74 has performed the act at least three times today without leaving the nest afterwards.
15:50 Mom in the act again
Incidentally, maintaining the pose for several minutes seems an energy-consuming exercise - it's not difficult to see that if a griffon spreads its wings and stays in the position for a few minutes, the degree to which the wings are stretched decreases gradually over time.
Nor are the vultures' wings likely to need drying. The Negev Desert is very much an arid area, no moisture can normally be observed on any of the vultures' body parts (except, occasionally, the beak after a feeding). Storks or cormorants may assume the horaltic posture primarily for this purpose, though probably not the Negev vultures.
Quite a few people I know are likely to place their bedclothes in a sunny spot on a warm morning. The reason for this, I'm told, is to bake off mites and bacteria. I find the analogy compelling.
But I'm just an amateur vulture watcher (and would therefore not attach too much weight to my own words) with little chance of ever observing one in my country. Just a few sightings have been reported in Poland over the last 100 years, even though the griffon vulture used to be resident in this territory still in the 19th century. I've had a few opportunities to watch griffons on both sides of the Pyrenees and in the Picos de Europa range, though. It's hard not to admire these majestic creatures.
22:02 The sleeping beauty
Learned something again. And also that they only lay one egg at a time.