In the Two Towers, the movie version, Gandalf obiviously wanted Theoden to take immediate actions against Sauraman. The thing that kicks in then is “What could Theoden do against Sauraman then?”
Unlike in the novel where Theoden still has a big company of riders, in the movie they seem only to number around 200 to 300 (if all they could muster for the Siege is 200) and Theoden is still pretty confident of taking on Sauraman, what could the Theoden in the movie do? What did Gandalf really want?
Of course, we could speculate to no ends - maybe Peter Jackson really slipped up here, who knows, but let make some wild and educated guesses!
My guess -
If met in a field battle, the Rohan Riders could dance circles around the half-orcs and Gandalf could use his pretty light trick to even up the odds.
Part of the plan is getting Eomer back.
Gandalf is foreseeing the Ents are attacking at the same time, so if Rohan is to attack Isengard, or to face Saurman at the same time, the chances of victory are much higher.