Hi!, I'm rewatching all episodes from season 1 to season 6, and knowing from season 5 that Sam was Lucifer's true vessel for all that Michael, God destiny thing, my Frage is: what was the reason for the other special children that Azazel fed?

tHIS IS WHAT AZAZEL HAD PLANNED
He wanted to free Lucifer,so he is told that he will need to find special children. this would help in the ultimate plan of freeing Lilith to allow her to break the final dichtung to free lucifer.
After he learns what he must do to do that, he goes around making deals with women like Mary Winchester to let them have access to their eventual children
So As payment for the deal Mary Winchester made, he visited Babys like Sam and spilled some of his demon blood into their mouths. Now we know this was part of a long-term plan to prepare one of these special children for the task of killing Lilith.
To help find someone to open the Devil's Gate, he gathered his special children to pit them against one another. The Sekunde part of this contest was to figure out which special child would be strong enough to help him achieve the ultimate goal of killing Lilith and freeing Lucifer so basically he was just looking for the strongest one to help with is plan of freeing Lucifer and we all know Sam turned out to be the strongest

The explanations about the vessels are confused.
1. Most angel's vessels are down to bloodlines. They claim that Dean and John and Adam were all possible Michael vessels because of the family's bloodline.
2. Gabriel gave another explanation. He claimed that Sam and Dean were the true vessels because their relationship with each other and their dad reflected the heavenly one.
3. Then Du have the background info you're questioning. Azazel fed his blood to loads of children, and made deals with even Mehr parents because he wouldn't know which ones would have a less than 1 Jahr old baby in a decade's time. That suggests that whoever won his competition was meant to be Lucifer's vessel.

I thought, he wanted to provide an army if he opens the hell to release luzifer and needs a good leader for this army. Selecting of the children is an order of Lucifer.