There were certainly numerous general gains to come from the expedtion
* A grasp of the true enormity of the Purchase and the physical breadth of the country.
* A grasp of the wide variety of physical environments in the Purchase territory - Great Plains, Rockies, Columbia Plateau, etc (of course there was information from other sources - but much of that extant information was dubious at best, and very incomplete)
* Confirmation that while a very arduous trek, the country could be crossed coast-to-coast
* A beginning catalog of minerals and their relative locations
* As noted earlier, a beginning catalog of flora and fauna.
* A glimpse of some of the Native American groups and their interaction with strangers.
* A beginning catalog of languages
* Basic meteorlogical study
* By being the official representatives of the US, Lewis & Clark put some substance to US claims over the region. I think that concept gave subsequent private US explorers credibility for their own expeditions, which were more commercial in purpose