While "elites" did exist, from the grenadiers in the left of the Line to named and favored regiments like pavlosk grenadiers or Scots Grey's, these seemed tied to being created as an extension of the societal elite "propping up" their own regiments. Some also earned their distinction in combat.
So what was the underlying causes for Napoleonic France to create an elite Force and increase it to such an extent the imperial guard was? Especially as the French state under the revolution lacked the societal elite that would say in England constitute the household guard.
And especially as in My assumption there isn't a big doctrinal precident for it.
And thirdly how it was justified as it drew resources and even ire from the regular army?
Was it Napoleons pet project?
A structural need?
A happenstance?
What often distinguished French units from their counterparts in the Napoleonic Wars prior to the invasion of Russia was their institutional and individual experience as soldiers. The success of Napoleon's campaigns and the massive scale of mobilisation in contrast to their Fredrecian modelled opponents left the French often experiencing far less casualties than their opponents. By the time Napoleon had decided to crown himself Emperor he formed a robust force that would often be considered the ace in the hole for the "Corsican Ogre" which was the Imperial Guard.
The Imperial guard was an elite force in an entirely different context to the units composed of noblemen that was considered Elite forces to the other continental powers, the Imperial Guard are considered to be a successor force of the Consular Guard and the Grenadiers of the Legislature which was an elite force of roughly 1000 experienced and loyal soldiers who were responsible for the personal protection of members of the Consular Executive and legislators in France, uniquely this Consular Guard actually refused to act against Napoleon's 18 Brumaire coup of 1799. In contrast to pedigree which was the basis of other nations elite regiments the requirements for the Imperial Guard was experience and stature.
The Imperial Guard of 1804 however is not the 100,000 man institution it grew into by the time of the Napoleonic foray into Imperial Russia, in 1804 it consisted of 8000 men who were the cream of the French crop, these were soldiers with the requirements to enter Napoleon's old guard being the most stringent of any European force by the time of their creation. The following were these requirements and if one is aware of the average experience and height for the period it is indeed very narrow criteria:
The men of the Old Guard and the Imperial Guard as a whole enjoyed numerous benefits reflecting upon their experience, skill and value to the Napoleonic regime in the form of higher wages and for the Old Guard in particular the express privilege to complaint in the presence of the Emperor of decisions or their conditions, this earned them among the Emperor's forces the nickname les Grognards or the Grumblers. These men were soldiers who had served with Napoleon from, in several cases his first battles. These soldiers were very seldom deployed into engagements unless deemed absolutely necessary by the Emperor himself.
However, the Imperial Guard's recruitment substantially expanded as Napoleon's conquests and military needs grew and in doing so the force as a whole lost it's experience and quality edge. The young guard for example consisted of the best soldiers found in every round of conscription in France, while these forces were the most disciplined and brave of the conscripts they did not enjoy the accured experience of multiple campaigns behind them. This in turn made them generally considered as a less elite force of the imperial guard than the old guard; but all the same an elite unit. The Middle Guard had a barrier of experience as the old guard did with the majority of the middle guardsmen having been veterans of Napoleon's campaigns from 1805 to 1809.
These soldiers were considered elite by virtue of their longstanding service or outstanding ability compared to the forces of the regular French army and were intentionally provided the best soldiers to cultivate a pedigree and increased performance. Many French soldiers prior to the invasion of Russia had fought in half a dozen campaigns and were reliable to carry out orders to the last man.