Charlie did it to make him stronger, lets take a look at the list of possible heirs.
Odysseus: Averages Joe, indecisive, possible he grownup with everything being handed to him and is apparently useless.
1st princess nobody: same as Odysseus.
Schneizel: the perfect heir for the throne, intelligent, ambitious and smooth with the ladies. Having older siblings probably help him by havening to compete for attention and have someone to lookup.
Cornelia: almost the same as Schneizel but with more focus on military, the main and most important difference is his sister, his weak point that is several times exploited by Lelouch and Schneizel; this leaves her as a good heir for the throne but with a weakness that makes her easy to manipulate.