To jump in a little as a new player as well.
Charge claims are related to the charge combat phase, none are tied to actually moving or having declared a charge - although you may give you opponent more claims if you do (ie. infantry charging cavalry).
Spending cards on charging is more about table position than getting a bonus in combat.
In theory, if you KNEW your opponent would spend a card, then yes you could say your cards were wasted. However, more likely is if you had the advantage and didn't declare charges they might be happy to sit tight and not charge either. If you wanted both of your lancers to fight the one UG you also need to declare charges for both - as again, if you declare only with one, your opponent could charge that and the other lancer would sit (although perhaps you could then move to a flank position - which to my newbie understanding is better than getting a 2-1 frontal fight). So the usefulness of spending cards for charges is contextual on the table position and what each player is trying to achieve.
Also, just incase youre not aware, lancers often get a free or forced charge (no card cost) with 3BW.