Napoleon wanted money for his grandiose plans for military conquest. He also realized the US adjoined the territory, really needed access to the Mississippi river and that it would be difficult for France to defend it against invasion. He probably did not realize how much land there actually was since much of it had never been explored. Napoleon also wanted the British to not gain a foothold in New Orleans and the Louisiana territory. He understood that it was a better situation for him to offer the land for sale in as much as he could not defend it and he'd rather have the US purchase it before Great Britain tried to take New Orleans and give Great Britain more territory, in addition to Canada, in North America.