Probably the problems with the fire of the third rank had not been so evident before the first battles of the 1813 campаign (Lutzen and Bautzen).
A battalion in 'column of divisions' - the traditional battle winning formation for the French infantry - would lose mass, reducing from 9 ranks deep to only six. That doesn't sound useful.
The rear ranks behind the first three produced only moral influence upon the first ranks and upon the enemy. There were no physical pressure of the rear ranks. This influence is difficult to estimate. Napoleon believed that 6 ranks would be enough.
Some French officers, for example, General Brenier, thought that the reduction of the formation depth from 3 to 2 ranks weakened the morale of the infantry.
General Chambray wrote that some officers believed that the three-rank formation was considerably more solid than the two-rank one, especially in repulsing enemy cavalry attacks. Chambray thought that the influence of the cadre (the officers and NCOs) is more important, and that in the the two-rank formation this influence is stronger than in the three-rank one.
Like others in this thread I suspect this has more to do with making shrinking battalions look bigger and take up more frontage on the battlefield.
This was probably the main reason for the order, and Napoleon clearly said that:
Cette formation a aussi cet avantage que la veille d'une bataille l'ennemi, n'en étant pas prévenu, évaluera l'armée qu'il a devant lui à des forces d'un tiers plus considérables qu'elles ne le sont.
"This formation also has an advantage that on the eve of a battle, the enemy, not being warned, will estimate the strength of the army in front of them by one third more considerable than it is [actually]."